brew install git

gistfile1.txt

==> Downloading http://git-core.googlecode.com/files/git-1.7.9.1.tar.gz
File already downloaded in /Users/rory/Library/Caches/Homebrew
/usr/bin/tar xf /Users/rory/Library/Caches/Homebrew/git-1.7.9.1.tar.gz
==> make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os -w -pipe -march=core2 -msse4.1 LDFLAGS= install
make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os -w -pipe -march=core2 -msse4.1 LDFLAGS= install
GIT_VERSION = 1.7.9.1
    * new build flags or prefix
    * new link flags
./generate-cmdlist.sh > common-cmds.h+ && mv common-cmds.h+ common-cmds.h
/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc -o hex.o -c -MF ./.depend/hex.o.d -MMD -MP  -Os -w -pipe -march=core2 -msse4.1 -I. -DUSE_ST_TIMESPEC -DNO_GETTEXT  -DHAVE_DEV_TTY -DSHA1_HEADER='<openssl/sha.h>'  -DNO_MEMMEM  hex.c
In file included from git-compat-util.h:239,
                 from cache.h:4,
                 from hex.c:1:
compat/bswap.h:12: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘default_swab32’
compat/bswap.h:25: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘git_bswap32’
In file included from cache.h:4,
                 from hex.c:1:
git-compat-util.h:319: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘read_in_full’
git-compat-util.h:436: error: expected declaration specifiers or ‘...’ before ‘off_t’
git-compat-util.h:437: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘xread’
git-compat-util.h:438: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘xwrite’
git-compat-util.h:440: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘*’ token
git-compat-util.h:446: error: expected ‘)’ before ‘len’
git-compat-util.h: In function ‘strtoul_ui’:
git-compat-util.h:499: error: ‘errno’ undeclared (first use in this function)
git-compat-util.h:499: error: (Each undeclared identifier is reported only once
git-compat-util.h:499: error: for each function it appears in.)
git-compat-util.h: In function ‘strtol_i’:
git-compat-util.h:512: error: ‘errno’ undeclared (first use in this function)
In file included from cache.h:5,
                 from hex.c:1:
strbuf.h: At top level:
strbuf.h:111: error: expected declaration specifiers or ‘...’ before ‘FILE’
strbuf.h:113: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘strbuf_read’
strbuf.h:117: error: expected declaration specifiers or ‘...’ before ‘FILE’
strbuf.h:118: error: expected declaration specifiers or ‘...’ before ‘FILE’
In file included from hex.c:1:
cache.h:21: error: expected specifier-qualifier-list before ‘z_stream’
cache.h: In function ‘create_ce_mode’:
cache.h:268: error: ‘S_IFLNK’ undeclared (first use in this function)
cache.h:269: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h:271: error: ‘S_IFREG’ undeclared (first use in this function)
cache.h: In function ‘ce_to_dtype’:
cache.h:291: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h: In function ‘canon_mode’:
cache.h:301: error: ‘S_IFREG’ undeclared (first use in this function)
cache.h:303: error: ‘S_IFLNK’ undeclared (first use in this function)
cache.h:305: error: ‘S_IFDIR’ undeclared (first use in this function)
cache.h: In function ‘object_type’:
cache.h:393: error: ‘S_IFMT’ undeclared (first use in this function)
cache.h: At top level:
cache.h:559: error: expected specifier-qualifier-list before ‘pid_t’
cache.h:775: error: expected declaration specifiers or ‘...’ before ‘time_t’
cache.h:969: error: expected specifier-qualifier-list before ‘off_t’
cache.h:978: error: expected specifier-qualifier-list before ‘off_t’
cache.h:996: error: expected specifier-qualifier-list before ‘off_t’
cache.h:1054: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1060: error: expected declaration specifiers or ‘...’ before ‘uint32_t’
cache.h:1061: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘nth_packed_object_offset’
cache.h:1062: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘find_pack_entry_one’
cache.h:1064: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1066: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1067: error: expected declaration specifiers or ‘...’ before ‘off_t’
cache.h:1091: error: expected specifier-qualifier-list before ‘off_t’
cache.h:1158: error: expected ‘)’ before ‘*’ token
cache.h:1167: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘read_in_full’
cache.h:1168: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘write_in_full’
cache.h:1169: error: expected ‘=’, ‘,’, ‘;’, ‘asm’ or ‘__attribute__’ before ‘write_str_in_full’
cache.h:1240: error: expected declaration specifiers or ‘...’ before ‘FILE’
make: *** [hex.o] Error 1
make: *** Waiting for unfinished jobs....
==> Exit Status: 2
http://github.com/mxcl/homebrew/blob/master/Library/Formula/git.rb#L47
==> Environment
HOMEBREW_VERSION: 0.8.1
HEAD: (none)
HOMEBREW_PREFIX: /usr/local
HOMEBREW_CELLAR: /usr/local/Cellar
Hardware: dual-core 64-bit penryn
OS X: 10.7.3
Kernel Architecture: x86_64
Xcode: 4.2
GCC-4.0: N/A
GCC-4.2: N/A
LLVM: build 2336
Clang: N/A
MacPorts or Fink? false
X11 installed? true
System Ruby: 1.8.7-249
/usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
Which Perl:   /usr/bin/perl
Which Python: /usr/bin/python
Which Ruby:   /usr/bin/ruby => /System/Library/Frameworks/Ruby.framework/Versions/1.8/usr/bin/ruby
==> Build Flags
CC: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CXX: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-g++ => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-g++-4.2
LD: /Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc => /Applications/Xcode.app/Contents/Developer/usr/llvm-gcc-4.2/bin/llvm-gcc-4.2
CFLAGS: -Os -w -pipe -march=core2 -msse4.1
CXXFLAGS: -Os -w -pipe -march=core2 -msse4.1
MAKEFLAGS: -j2

Error: Failed executing: make prefix=/usr/local/Cellar/git/1.7.9.1 CC=/Applications/Xcode.app/Contents/Developer/usr/bin/llvm-gcc CFLAGS=-Os\ -w\ -pipe\ -march=core2\ -msse4.1 LDFLAGS= install
These existing issues may help you:
    https://github.com/mxcl/homebrew/issues/8977
    https://github.com/mxcl/homebrew/issues/9023
    https://github.com/mxcl/homebrew/issues/9618
    https://github.com/mxcl/homebrew/issues/10081
    https://github.com/mxcl/homebrew/issues/10225
Otherwise, please report the bug:
    https://github.com/mxcl/homebrew/wiki/checklist-before-filing-a-new-issue

It was in reference to this issue here Homebrew/legacy-homebrew#10244 which seems to have been resolved.

Perfect, thanks I ran into the same issue while migrating to a new machine.