checkout: rearrange update_refs_for_switch for clarity

Take care of simple, exceptional cases before the meat of the "check
out by branch name" code begins.  After this change, the function
vaguely follows the following pseudocode:

	if (-B or -b)
		create branch;
	if (plain "git checkout" or "git checkout HEAD")
		;
	else if (--detach or checking out by non-branch commit name)
		detach HEAD;
	else if (checking out by branch name)
		attach HEAD;

One nice side benefit is to make it possible to remove handling of
the --detach option from outside switch_branches.

Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/checkout.c

@@ -542,7 +542,17 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 	strbuf_addf(&msg, "checkout: moving from %s to %s",
 		    old_desc ? old_desc : "(invalid)", new->name);
 
-	if (new->path) {
+	if (!strcmp(new->name, "HEAD") && !new->path && !opts->force_detach) {
+		/* Nothing to do. */
+	} else if (opts->force_detach || !new->path) {	/* No longer on any branch. */
+		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
+			   REF_NODEREF, DIE_ON_ERR);
+		if (!opts->quiet) {
+			if (old->path && advice_detached_head)
+				detach_advice(old->path, new->name);
+			describe_detached_head("HEAD is now at", new->commit);
+		}
+	} else if (new->path) {	/* Switch branches. */
 		create_symref("HEAD", new->path, msg.buf);
 		if (!opts->quiet) {
 			if (old->path && !strcmp(new->path, old->path))
@@ -564,14 +574,6 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 			if (!file_exists(ref_file) && file_exists(log_file))
 				remove_path(log_file);
 		}
-	} else if (opts->force_detach || strcmp(new->name, "HEAD")) {
-		update_ref(msg.buf, "HEAD", new->commit->object.sha1, NULL,
-			   REF_NODEREF, DIE_ON_ERR);
-		if (!opts->quiet) {
-			if (old->path && advice_detached_head)
-				detach_advice(old->path, new->name);
-			describe_detached_head("HEAD is now at", new->commit);
-		}
 	}
 	remove_branch_state();
 	strbuf_release(&msg);
@@ -679,7 +681,6 @@ static const char *unique_tracking_name(const char *name)
 
 static int parse_branchname_arg(int argc, const char **argv,
 				int dwim_new_local_branch_ok,
-				int force_detach,
 				struct branch_info *new,
 				struct tree **source_tree,
 				unsigned char rev[20],
@@ -756,8 +757,7 @@ static int parse_branchname_arg(int argc, const char **argv,
 	new->name = arg;
 	setup_branch_path(new);
 
-	if (!force_detach &&
-	    check_ref_format(new->path) == CHECK_REF_FORMAT_OK &&
+	if (check_ref_format(new->path) == CHECK_REF_FORMAT_OK &&
 	    resolve_ref(new->path, branch_rev, 1, NULL))
 		hashcpy(rev, branch_rev);
 	else
@@ -906,8 +906,7 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 			dwim_new_local_branch &&
 			opts.track == BRANCH_TRACK_UNSPECIFIED &&
 			!opts.new_branch;
-		int n = parse_branchname_arg(argc, argv,
-				dwim_ok, opts.force_detach,
+		int n = parse_branchname_arg(argc, argv, dwim_ok,
 				&new, &source_tree, rev, &opts.new_branch);
 		argv += n;
 		argc -= n;