Add "git-sh-setup-script" for common git shell script setup

It sets up the normal git environment variables and a few helper
functions (currently just "die()"), and returns ok if it all looks like
a git archive.  So use it something like

	. git-sh-setup-script || die "Not a git archive"

to make the rest of the git scripts more careful and readable.

git-fetch-script

@@ -9,8 +9,8 @@ if [ "$2" = "tag" ]; then
 	destination="$merge_name"
 fi
 
-: ${GIT_DIR=.git}
-: ${GIT_OBJECT_DIRECTORY="${SHA1_FILE_DIRECTORY-"$GIT_DIR/objects"}"}
+. git-sh-setup-script || die "Not a git archive"
+
 TMP_HEAD="$GIT_DIR/TMP_HEAD"
 
 case "$merge_repo" in