builtin-grep: wildcard pathspec fixes

builtin-grep: wildcard pathspec fixes

This tweaks the pathspec wildcard used in builtin-grep to match
that of ls-files.  With this:

	git grep -e DEBUG -- '*/Kconfig*'

would work like the shell script version, and you could even do:

	git grep -e DEBUG --cached -- '*/Kconfig*' ;# from index
	git grep -e DEBUG v2.6.12 -- '*/Kconfig*' ;# from rev

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