add NO_EXTERNAL_GREP build option

Previously, we just chose whether to allow external grep
based on the __unix__ define. However, there are systems
which define this macro but which have an inferior group
(e.g., one that does not support all options used by t7002).
This allows users to accept the potential speed penalty to
get a more consistent grep experience (and to pass the
testsuite).

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin-grep.c

@@ -12,6 +12,14 @@
 #include "builtin.h"
 #include "grep.h"
 
+#ifndef NO_EXTERNAL_GREP
+#ifdef __unix__
+#define NO_EXTERNAL_GREP 0
+#else
+#define NO_EXTERNAL_GREP 1
+#endif
+#endif
+
 /*
  * git grep pathspecs are somewhat different from diff-tree pathspecs;
  * pathname wildcards are allowed.
@@ -153,7 +161,7 @@ static int grep_file(struct grep_opt *opt, const char *filename)
 	return i;
 }
 
-#ifdef __unix__
+#if !NO_EXTERNAL_GREP
 static int exec_grep(int argc, const char **argv)
 {
 	pid_t pid;
@@ -372,7 +380,7 @@ static int grep_cache(struct grep_opt *opt, const char **paths, int cached)
 	int nr;
 	read_cache();
 
-#ifdef __unix__
+#if !NO_EXTERNAL_GREP
 	/*
 	 * Use the external "grep" command for the case where
 	 * we grep through the checked-out files. It tends to