sh-setup: add new peel_committish() helper

The normal way to check whether a certain revision resolves to a valid
commit is:

  $ git rev-parse --verify $REV^0

Unfortunately, this does not work when $REV is of the type :/quuxery.
Write a helper to work around this limitation.

Suggested-by: Junio C Hamano <gitster@pobox.com>
Signed-off-by: Ramkumar Ramachandra <artagnon@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Author: artagnon

git-sh-setup.sh

@@ -313,3 +313,15 @@ then
 	}
 	: ${GIT_OBJECT_DIRECTORY="$GIT_DIR/objects"}
 fi
+
+peel_committish () {
+	case "$1" in
+	:/*)
+		peeltmp=$(git rev-parse --verify "$1") &&
+		git rev-parse --verify "${peeltmp}^0"
+		;;
+	*)
+		git rev-parse --verify "${1}^0"
+		;;
+	esac
+}