git-grep: do not die upon -F/-P when grep.extendedRegexp is set.

The previous one made "git grep -P" fail when grep.extendedRegexp is
enabled.  That is a no-starter.  The option on the command line should
just make the command ignore the configured default.  The handling of "-F"
in the existing code has the same problem.

Instead of saying -G/-F/-E/-P incompatible with each other, just allow
the last one win.  That way, you can have "[alias] gr = grep -P" and
use Pcre for everyday work e.g. "git gr ':i?foo'", and append -G to the
aliased command line to override it e.g. "git gr -G '[Ff][Oo][Oo]'".

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

builtin/grep.c

@@ -753,6 +753,15 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 	int i;
 	int dummy;
 	int use_index = 1;
+	enum {
+		pattern_type_unspecified = 0,
+		pattern_type_bre,
+		pattern_type_ere,
+		pattern_type_fixed,
+		pattern_type_pcre,
+	};
+	int pattern_type = pattern_type_unspecified;
+
 	struct option options[] = {
 		OPT_BOOLEAN(0, "cached", &cached,
 			"search in index instead of in the work tree"),
@@ -774,15 +783,18 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			"descend at most <depth> levels", PARSE_OPT_NONEG,
 			NULL, 1 },
 		OPT_GROUP(""),
-		OPT_BIT('E', "extended-regexp", &opt.regflags,
-			"use extended POSIX regular expressions", REG_EXTENDED),
-		OPT_NEGBIT('G', "basic-regexp", &opt.regflags,
-			"use basic POSIX regular expressions (default)",
-			REG_EXTENDED),
-		OPT_BOOLEAN('F', "fixed-strings", &opt.fixed,
-			"interpret patterns as fixed strings"),
-		OPT_BOOLEAN('P', "perl-regexp", &opt.pcre,
-				"use Perl-compatible regular expressions"),
+		OPT_SET_INT('E', "extended-regexp", &pattern_type,
+			    "use extended POSIX regular expressions",
+			    pattern_type_ere),
+		OPT_SET_INT('G', "basic-regexp", &pattern_type,
+			    "use basic POSIX regular expressions (default)",
+			    pattern_type_bre),
+		OPT_SET_INT('F', "fixed-strings", &pattern_type,
+			    "interpret patterns as fixed strings",
+			    pattern_type_fixed),
+		OPT_SET_INT('P', "perl-regexp", &pattern_type,
+			    "use Perl-compatible regular expressions",
+			    pattern_type_pcre),
 		OPT_GROUP(""),
 		OPT_BOOLEAN('n', "line-number", &opt.linenum, "show line numbers"),
 		OPT_NEGBIT('h', NULL, &opt.pathname, "don't show filenames", 1),
@@ -888,6 +900,28 @@ int cmd_grep(int argc, const char **argv, const char *prefix)
 			     PARSE_OPT_KEEP_DASHDASH |
 			     PARSE_OPT_STOP_AT_NON_OPTION |
 			     PARSE_OPT_NO_INTERNAL_HELP);
+	switch (pattern_type) {
+	case pattern_type_fixed:
+		opt.fixed = 1;
+		opt.pcre = 0;
+		break;
+	case pattern_type_bre:
+		opt.fixed = 0;
+		opt.pcre = 0;
+		opt.regflags &= ~REG_EXTENDED;
+		break;
+	case pattern_type_ere:
+		opt.fixed = 0;
+		opt.pcre = 0;
+		opt.regflags |= REG_EXTENDED;
+		break;
+	case pattern_type_pcre:
+		opt.fixed = 0;
+		opt.pcre = 1;
+		break;
+	default:
+		break; /* nothing */
+	}
 
 	if (use_index && !startup_info->have_repository)
 		/* die the same way as if we did it at the beginning */
@@ -925,12 +959,8 @@ 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.pcre) && opt.fixed)
-		die(_("cannot mix --fixed-strings and regexp"));
 
 #ifndef NO_PTHREADS
 	if (online_cpus() == 1 || !grep_threads_ok(&opt))