Port of hgeditor to work with git

giteditor

#!/bin/sh -x
#
# based on http://selenic.com/git/raw-file/tip/hgeditor
# and http://stackoverflow.com/questions/4750148/git-show-index-diff-in-commit-message-as-comment
#
# This is an example of using GITEDITOR to create of diff to review the
# changes while commiting.
#
# If you want to pass your favourite editor some other parameters
# only for Git, modify this:
case "${EDITOR}" in
    "")
        EDITOR="vi"
        ;;
    emacs)
        EDITOR="$EDITOR -nw"
        ;;
    gvim|vim)
        EDITOR="$EDITOR -f -O"
        ;;
esac


GITTMP=""
cleanup_exit() {
    rm -rf "$GITTMP"
}

# Remove temporary files even if we get interrupted
trap "cleanup_exit" 0 # normal exit
trap "exit 255" HUP INT QUIT ABRT TERM

GITTMP=$(mktemp -d ${TMPDIR-/tmp}/giteditor.XXXXXX)
[ x$GITTMP != x -a -d $GITTMP ] || {
  echo "Could not create temporary directory! Exiting." 1>&2
  exit 1
}

(
    git diff --no-ext-diff --staged -p >> "$GITTMP/diff"
)

cat "$1" > "$GITTMP/msg"

MD5=$(which md5sum 2>/dev/null) || \
    MD5=$(which md5 2>/dev/null)
[ -x "${MD5}" ] && CHECKSUM=`${MD5} "$GITTMP/msg"`
if [ -s "$GITTMP/diff" ]; then
    $EDITOR "$GITTMP/msg" "$GITTMP/diff" || exit $?
else
    $EDITOR "$GITTMP/msg" || exit $?
fi
[ -x "${MD5}" ] && (echo "$CHECKSUM" | ${MD5} -c >/dev/null 2>&1 && exit 13)

mv "$GITTMP/msg" "$1"

exit $?