grep: add --threads=<num> option and grep.threads configuration

"git grep" can now be configured (or told from the command line) how
many threads to use when searching in the working tree files.

Signed-off-by: Victor Leschuk <vleschuk@accesssoftek.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1310,6 +1310,7 @@ _git_grep ()
 			--full-name --line-number
 			--extended-regexp --basic-regexp --fixed-strings
 			--perl-regexp
+			--threads
 			--files-with-matches --name-only
 			--files-without-match
 			--max-depth