Clarify that git-grep(1) searches only tracked files, and that each
<pathspec> is a pathspec, as in any other ordinary git commands.
Add an example to show a simple use case for searching all .c and .h
files in the current directory and below.
Signed-off-by: Mark Lodato <lodatom@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>