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

Makefile

@@ -148,6 +148,9 @@ all::
 # is a simplified version of the merge sort used in glibc. This is
 # recommended if Git triggers O(n^2) behavior in your platform's qsort().
 #
+# Define NO_EXTERNAL_GREP if you don't want "git grep" to ever call
+# your external grep (e.g., if your system lacks grep, if its grep is
+# broken, or spawning external process is slower than built-in grep git has).
 
 GIT-VERSION-FILE: .FORCE-GIT-VERSION-FILE
 	@$(SHELL_PATH) ./GIT-VERSION-GEN
@@ -760,6 +763,9 @@ endif
 ifdef DIR_HAS_BSD_GROUP_SEMANTICS
 	COMPAT_CFLAGS += -DDIR_HAS_BSD_GROUP_SEMANTICS
 endif
+ifdef NO_EXTERNAL_GREP
+	BASIC_CFLAGS += -DNO_EXTERNAL_GREP
+endif
 
 ifeq ($(TCLTK_PATH),)
 NO_TCLTK=NoThanks

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