branch.c: replace `git_config()` with `git_config_get_string()

Use `git_config_get_string()` instead of `git_config()` to take advantage of
the config-set API which provides a cleaner control flow. While we are at
it, return -1 if we find no value for the queried variable. Original code
returned 0 for all cases, which was checked by `add_branch_desc()` in
fmt-merge-msg.c resulting in addition of a spurious newline to the `out`
strbuf. Now, the newline addition is skipped as -1 is returned to the caller
if no value is found.

Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Reviewed-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

branch.c

@@ -140,30 +140,17 @@ static int setup_tracking(const char *new_ref, const char *orig_ref,
 	return 0;
 }
 
-struct branch_desc_cb {
-	const char *config_name;
-	const char *value;
-};
-
-static int read_branch_desc_cb(const char *var, const char *value, void *cb)
-{
-	struct branch_desc_cb *desc = cb;
-	if (strcmp(desc->config_name, var))
-		return 0;
-	free((char *)desc->value);
-	return git_config_string(&desc->value, var, value);
-}
-
 int read_branch_desc(struct strbuf *buf, const char *branch_name)
 {
-	struct branch_desc_cb cb;
+	char *v = NULL;
 	struct strbuf name = STRBUF_INIT;
 	strbuf_addf(&name, "branch.%s.description", branch_name);
-	cb.config_name = name.buf;
-	cb.value = NULL;
-	git_config(read_branch_desc_cb, &cb);
-	if (cb.value)
-		strbuf_addstr(buf, cb.value);
+	if (git_config_get_string(name.buf, &v)) {
+		strbuf_release(&name);
+		return -1;
+	}
+	strbuf_addstr(buf, v);
+	free(v);
 	strbuf_release(&name);
 	return 0;
 }