git-grep: Bail out when -P is used with -F or -E

This patch makes git-grep die() when -P is used on command line together
with -E/--extended-regexp or -F/--fixed-strings.

This also makes it bail out when grep.extendedRegexp is enabled.

But `git grep -G -P pattern` and `git grep -E -G -P pattern` still work
because -G and -E set opts.regflags during parse_options() and there is
no way to detect `-G` or `-E -G`.

Signed-off-by: Michał Kiedrowicz <michal.kiedrowicz@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/grep.c

@@ -925,9 +925,11 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 
 	if (!opt.pattern_list)
 		die(_("no pattern given."));
+	if (opt.regflags != REG_NEWLINE && opt.pcre)
+		die(_("cannot mix --extended-regexp and --perl-regexp"));
 	if (!opt.fixed && opt.ignore_case)
 		opt.regflags |= REG_ICASE;
-	if ((opt.regflags != REG_NEWLINE) && opt.fixed)
+	if ((opt.regflags != REG_NEWLINE || opt.pcre) && opt.fixed)
 		die(_("cannot mix --fixed-strings and regexp"));
 
 #ifndef NO_PTHREADS

t/t7810-grep.sh

@@ -637,4 +637,20 @@ test_expect_success LIBPCRE 'grep -P -w pattern' '
 	test_cmp expected actual
 '
 
+test_expect_success LIBPCRE 'grep -P -F returns error' '
+	test_expect_code 128 git grep -P -F main
+'
+
+test_expect_success LIBPCRE 'grep -P -E returns error' '
+	test_expect_code 128 git grep -P -E main
+'
+
+test_expect_failure LIBPCRE 'grep -P -G returns error' '
+	test_expect_code 128 git grep -P -G main
+'
+
+test_expect_failure LIBPCRE 'grep -P -E -G returns error' '
+	test_expect_code 128 git grep -P -E -G main
+'
+
 test_done