config: create a function to format section headers

Factor out the logic which creates section headers in the config file,
e.g. the 'branch.foo' key will be turned into '[branch "foo"]'.

This introduces no function changes, but is needed for a later change
which adds support for copying branch sections in the config file.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Ramsay Jones <ramsay@ramsayjones.plus.com>
Signed-off-by: Sahil Dua <sahildua2305@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

config.c

@@ -2169,10 +2169,10 @@ static int write_error(const char *filename)
 	return 4;
 }
 
-static int store_write_section(int fd, const char *key)
+static struct strbuf store_create_section(const char *key)
 {
 	const char *dot;
-	int i, success;
+	int i;
 	struct strbuf sb = STRBUF_INIT;
 
 	dot = memchr(key, '.', store.baselen);
@@ -2188,6 +2188,15 @@ static int store_write_section(int fd, const char *key)
 		strbuf_addf(&sb, "[%.*s]\n", store.baselen, key);
 	}
 
+	return sb;
+}
+
+static int store_write_section(int fd, const char *key)
+{
+	int success;
+
+	struct strbuf sb = store_create_section(key);
+
 	success = write_in_full(fd, sb.buf, sb.len) == sb.len;
 	strbuf_release(&sb);