merge: split write_merge_state in two

write_merge_state() writes out the merge heads, mode, and msg. But we
may want to write out heads, mode without the msg. So, split out heads
(+mode) into a separate function write_merge_heads() that is called by
write_merge_state().

No funtional change so far, except when these non-atomic writes are
interrupted: we write heads-mode-msg now when we used to write
heads-msg-mode.

Signed-off-by: Michael J Gruber <git@grubix.eu>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/merge.c

@@ -906,7 +906,7 @@ static int setup_with_upstream(const char ***argv)
 	return i;
 }
 
-static void write_merge_state(struct commit_list *remoteheads)
+static void write_merge_heads(struct commit_list *remoteheads)
 {
 	struct commit_list *j;
 	struct strbuf buf = STRBUF_INIT;
@@ -922,8 +922,6 @@ static void write_merge_state(struct commit_list *remoteheads)
 		strbuf_addf(&buf, "%s\n", oid_to_hex(oid));
 	}
 	write_file_buf(git_path_merge_head(), buf.buf, buf.len);
-	strbuf_addch(&merge_msg, '\n');
-	write_file_buf(git_path_merge_msg(), merge_msg.buf, merge_msg.len);
 
 	strbuf_reset(&buf);
 	if (fast_forward == FF_NO)
@@ -931,6 +929,13 @@ static void write_merge_state(struct commit_list *remoteheads)
 	write_file_buf(git_path_merge_mode(), buf.buf, buf.len);
 }
 
+static void write_merge_state(struct commit_list *remoteheads)
+{
+	write_merge_heads(remoteheads);
+	strbuf_addch(&merge_msg, '\n');
+	write_file_buf(git_path_merge_msg(), merge_msg.buf, merge_msg.len);
+}
+
 static int default_edit_option(void)
 {
 	static const char name[] = "GIT_MERGE_AUTOEDIT";