make config --add behave correctly for empty and NULL values

Currently if we have a config file like,
[foo]
        baz
        bar =

and we try something like, "git config --add foo.baz roll", Git will
segfault. Moreover, for "git config --add foo.bar roll", it will
overwrite the original value instead of appending after the existing
empty value.

The problem lies with the regexp used for simulating --add in
`git_config_set_multivar_in_file()`, "^$", which in ideal case should
not match with any string but is true for empty strings. Instead use a
regexp like "a^" which can not be true for any string, empty or not.

For removing the segfault add a check for NULL values in `matches()` in
config.c.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -1233,7 +1233,7 @@ static int matches(const char *key, const char *value)
 	return !strcmp(key, store.key) &&
 		(store.value_regex == NULL ||
 		 (store.do_not_match ^
-		  !regexec(store.value_regex, value, 0, NULL, 0)));
+		  (value && !regexec(store.value_regex, value, 0, NULL, 0))));
 }
 
 static int store_aux(const char *key, const char *value, void *cb)