parse_config_key: allow matching single-level config

The parse_config_key() function was introduced to make it
easier to match "section.subsection.key" variables. It also
handles the simpler "section.key", and the caller is
responsible for distinguishing the two from its
out-parameters.

Most callers who _only_ want "section.key" would just use a
strcmp(var, "section.key"), since there is no parsing
required. However, they may still use parse_config_key() if
their "section" variable isn't a constant (an example of
this is in parse_hide_refs_config).

Using the parse_config_key is a bit clunky, though:

  const char *subsection;
  int subsection_len;
  const char *key;

  if (!parse_config_key(var, section, &subsection, &subsection_len, &key) &&
      !subsection) {
	  /* matched! */
  }

Instead, let's treat a NULL subsection as an indication that
the caller does not expect one. That lets us write:

  const char *key;

  if (!parse_config_key(var, section, NULL, NULL, &key)) {
	  /* matched! */
  }

Existing callers should be unaffected, as passing a NULL
subsection would currently segfault.

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

config.c

@@ -2547,10 +2547,14 @@ int parse_config_key(const char *var,
 
 	/* Did we have a subsection at all? */
 	if (dot == var) {
-		*subsection = NULL;
-		*subsection_len = 0;
+		if (subsection) {
+			*subsection = NULL;
+			*subsection_len = 0;
+		}
 	}
 	else {
+		if (!subsection)
+			return -1;
 		*subsection = var + 1;
 		*subsection_len = dot - *subsection;
 	}