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-prune-script

@@ -1,4 +1,7 @@
 #!/bin/sh
+
+. git-sh-setup-script || die "Not a git archive"
+
 dryrun=
 while case "$#" in 0) break ;; esac
 do
@@ -11,9 +14,6 @@ do
     shift;
 done
 
-: ${GIT_DIR=.git}
-: ${GIT_OBJECT_DIRECTORY="${SHA1_FILE_DIRECTORY-"$GIT_DIR/objects"}"}
-
 git-fsck-cache --cache --unreachable "$@" |
 sed -ne '/unreachable /{
     s/unreachable [^ ][^ ]* //