grep: prepare to run outside of a work tree

This moves the call to setup_git_directory() for running "grep" from
the "git" wrapper to the implementation of the "grep" subcommand.  A
new variable "use_index" is always true at this stage in the series,
and when it is on, we require that we are in a directory that is under
git control.  To make sure we die the same way, we make a second call
into setup_git_directory() when we detect this situation.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git.c

@@ -317,7 +317,7 @@ static void handle_internal_command(int argc, const char **argv)
 		{ "fsck-objects", cmd_fsck, RUN_SETUP },
 		{ "gc", cmd_gc, RUN_SETUP },
 		{ "get-tar-commit-id", cmd_get_tar_commit_id },
-		{ "grep", cmd_grep, RUN_SETUP | USE_PAGER },
+		{ "grep", cmd_grep, USE_PAGER },
 		{ "help", cmd_help },
 		{ "init", cmd_init_db },
 		{ "init-db", cmd_init_db },