optimize vcs_info on Cygwin

gistfile1.diff

--- /usr/share/zsh/4.3.12/functions/VCS_INFO_bydir_detect       2011-08-08 04:59:41.001000000 +0900
+++ VCS_INFO_bydir_detect       2012-01-19 21:52:35.293434600 +0900
@@ -6,7 +6,7 @@
 local dirname=$1
 local basedir="." realbasedir file
 
-realbasedir="$(VCS_INFO_realpath ${basedir})"
+realbasedir="${basedir:A}"
 while [[ ${realbasedir} != '/' ]]; do
     [[ -r ${realbasedir} ]] || return 1
     if [[ -n ${vcs_comm[detect_need_file]} ]] ; then
@@ -20,7 +20,7 @@
     fi
 
     basedir=${basedir}/..
-    realbasedir="$(VCS_INFO_realpath ${basedir})"
+    realbasedir="${basedir:A}"
 done
 
 [[ ${realbasedir} == "/" ]] && return 1