Allow checkout -B <current-branch> to update the current branch

When on master, "git checkout -B master <commit>" is a more natural way to
say "git reset --keep <commit>", which was originally invented for the
exact purpose of moving to the named commit while keeping the local changes
around.

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

branch.c

@@ -160,7 +160,8 @@ int validate_new_branchname(const char *name, struct strbuf *ref,
 
 void create_branch(const char *head,
 		   const char *name, const char *start_name,
-		   int force, int reflog, enum branch_track track)
+		   int force, int reflog, int clobber_head,
+		   enum branch_track track)
 {
 	struct ref_lock *lock = NULL;
 	struct commit *commit;
@@ -175,7 +176,8 @@ void create_branch(const char *head,
 		explicit_tracking = 1;
 
 	if (validate_new_branchname(name, &ref, force,
-				    track == BRANCH_TRACK_OVERRIDE)) {
+				    track == BRANCH_TRACK_OVERRIDE ||
+				    clobber_head)) {
 		if (!force)
 			dont_change_ref = 1;
 		else

branch.h

@@ -13,7 +13,8 @@
  * branch for (if any).
  */
 void create_branch(const char *head, const char *name, const char *start_name,
-		   int force, int reflog, enum branch_track track);
+		   int force, int reflog,
+		   int clobber_head, enum branch_track track);
 
 /*
  * Validates that the requested branch may be created, returning the

builtin/branch.c

@@ -737,7 +737,7 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		if (kinds != REF_LOCAL_BRANCH)
 			die(_("-a and -r options to 'git branch' do not make sense with a branch name"));
 		create_branch(head, argv[0], (argc == 2) ? argv[1] : head,
-			      force_create, reflog, track);
+			      force_create, reflog, 0, track);
 	} else
 		usage_with_options(builtin_branch_usage, options);
 

builtin/checkout.c

@@ -540,7 +540,9 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 		else
 			create_branch(old->name, opts->new_branch, new->name,
 				      opts->new_branch_force ? 1 : 0,
-				      opts->new_branch_log, opts->track);
+				      opts->new_branch_log,
+				      opts->new_branch_force ? 1 : 0,
+				      opts->track);
 		new->name = opts->new_branch;
 		setup_branch_path(new);
 	}
@@ -565,8 +567,12 @@ static void update_refs_for_switch(struct checkout_opts *opts,
 		create_symref("HEAD", new->path, msg.buf);
 		if (!opts->quiet) {
 			if (old->path && !strcmp(new->path, old->path)) {
-				fprintf(stderr, _("Already on '%s'\n"),
-					new->name);
+				if (opts->new_branch_force)
+					fprintf(stderr, _("Reset branch '%s'\n"),
+						new->name);
+				else
+					fprintf(stderr, _("Already on '%s'\n"),
+						new->name);
 			} else if (opts->new_branch) {
 				if (opts->branch_exists)
 					fprintf(stderr, _("Switched to and reset branch '%s'\n"), new->name);
@@ -1057,7 +1063,8 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 		struct strbuf buf = STRBUF_INIT;
 
 		opts.branch_exists = validate_new_branchname(opts.new_branch, &buf,
-							     !!opts.new_branch_force, 0);
+							     !!opts.new_branch_force,
+							     !!opts.new_branch_force);
 
 		strbuf_release(&buf);
 	}

t/t2018-checkout-branch.sh

@@ -189,12 +189,13 @@ test_expect_success 'checkout -b <describe>' '
 	test_cmp expect actual
 '
 
-test_expect_success 'checkout -B to the current branch fails before merging' '
+test_expect_success 'checkout -B to the current branch works' '
 	git checkout branch1 &&
+	git checkout -B branch1-scratch &&
+
 	setup_dirty_mergeable &&
-	git commit -mfooble &&
-	test_must_fail git checkout -B branch1 initial &&
-	test_must_fail test_dirty_mergeable
+	git checkout -B branch1-scratch initial &&
+	test_dirty_mergeable
 '
 
 test_done