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>

t/t1303-wacky-config.sh

@@ -111,4 +111,24 @@ test_expect_success 'unset many entries' '
 	test_must_fail git config section.key
 '
 
+test_expect_success '--add appends new value after existing empty value' '
+	cat >expect <<-\EOF &&
+
+
+	fool
+	roll
+	EOF
+	cp .git/config .git/config.old &&
+	test_when_finished "mv .git/config.old .git/config" &&
+	cat >.git/config <<-\EOF &&
+	[foo]
+		baz
+		baz =
+		baz = fool
+	EOF
+	git config --add foo.baz roll &&
+	git config --get-all foo.baz >output &&
+	test_cmp expect output
+'
+
 test_done