Merge branch 'rs/use-enhanced-bre-on-macos'

Newer regex library macOS stopped enabling GNU-like enhanced BRE,
where '\(A\|B\)' works as alternation, unless explicitly asked with
the REG_ENHANCED flag.  "git grep" now can be compiled to do so, to
retain the old behaviour.

* rs/use-enhanced-bre-on-macos:
  use enhanced basic regular expressions on macOS

compat/regcomp_enhanced.c

@@ -0,0 +1,9 @@
+#include "../git-compat-util.h"
+#undef regcomp
+
+int git_regcomp(regex_t *preg, const char *pattern, int cflags)
+{
+	if (!(cflags & REG_EXTENDED))
+		cflags |= REG_ENHANCED;
+	return regcomp(preg, pattern, cflags);
+}