config: teach "git -c" to recognize an empty string

In a config file, you can do:

  [foo]
  bar

to turn the "foo.bar" boolean flag on, and you can do:

  [foo]
  bar=

to set "foo.bar" to the empty string. However, git's "-c"
parameter treats both:

  git -c foo.bar

and

  git -c foo.bar=

as the boolean flag, and there is no way to set a variable
to the empty string. This patch enables the latter form to
do that.

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

Documentation/git.txt

@@ -429,6 +429,11 @@ example the following invocations are equivalent:
 	given will override values from configuration files.
 	The <name> is expected in the same format as listed by
 	'git config' (subkeys separated by dots).
++
+Note that omitting the `=` in `git -c foo.bar ...` is allowed and sets
+`foo.bar` to the boolean true value (just like `[foo]bar` would in a
+config file). Including the equals but with an empty value (like `git -c
+foo.bar= ...`) sets `foo.bar` to the empty string.
 
 --exec-path[=<path>]::
 	Path to wherever your core Git programs are installed.

config.c

@@ -164,19 +164,27 @@ void git_config_push_parameter(const char *text)
 int git_config_parse_parameter(const char *text,
 			       config_fn_t fn, void *data)
 {
+	const char *value;
 	struct strbuf **pair;
+
 	pair = strbuf_split_str(text, '=', 2);
 	if (!pair[0])
 		return error("bogus config parameter: %s", text);
-	if (pair[0]->len && pair[0]->buf[pair[0]->len - 1] == '=')
+
+	if (pair[0]->len && pair[0]->buf[pair[0]->len - 1] == '=') {
 		strbuf_setlen(pair[0], pair[0]->len - 1);
+		value = pair[1] ? pair[1]->buf : "";
+	} else {
+		value = NULL;
+	}
+
 	strbuf_trim(pair[0]);
 	if (!pair[0]->len) {
 		strbuf_list_free(pair);
 		return error("bogus config parameter: %s", text);
 	}
 	lowercase(pair[0]->buf);
-	if (fn(pair[0]->buf, pair[1] ? pair[1]->buf : NULL, data) < 0) {
+	if (fn(pair[0]->buf, value, data) < 0) {
 		strbuf_list_free(pair);
 		return -1;
 	}

t/t1300-repo-config.sh

@@ -1006,6 +1006,17 @@ test_expect_success 'git -c "key=value" support' '
 	test_must_fail git -c name=value config core.name
 '
 
+# We just need a type-specifier here that cares about the
+# distinction internally between a NULL boolean and a real
+# string (because most of git's internal parsers do care).
+# Using "--path" works, but we do not otherwise care about
+# its semantics.
+test_expect_success 'git -c can represent empty string' '
+	echo >expect &&
+	git -c foo.empty= config --path foo.empty >actual &&
+	test_cmp expect actual
+'
+
 test_expect_success 'key sanity-checking' '
 	test_must_fail git config foo=bar &&
 	test_must_fail git config foo=.bar &&