git-grep: show pathnames relative to the current directory

git-grep: show pathnames relative to the current directory

By default, the command shows pathnames relative to the current
directory.  Use --full-name (the same flag to do so in ls-files)
if you want to see the full pathname relative to the project root.

This makes it very pleasant to run in Emacs compilation (or
"grep-find") buffer.

Signed-off-by: Junio C Hamano <junkio@cox.net>