rebase: refuse to switch to branch already checked out elsewhere

The invocation "git rebase <upstream> <branch>" switches to <branch>
before performing the rebase operation. However, unlike git-switch,
git-checkout, and git-worktree which all refuse to switch to a branch
that is already checked out in some other worktree, git-rebase switches
to <branch> unconditionally. Curb this careless behavior by making
git-rebase also refuse to switch to a branch checked out elsewhere.

Reported-by: Mike Hommey <mh@glandium.org>
Signed-off-by: Eric Sunshine <sunshine@sunshineco.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

builtin/rebase.c

@@ -1953,10 +1953,11 @@ int cmd_rebase(int argc, const char **argv, const char *prefix)
 		/* Is it a local branch? */
 		strbuf_reset(&buf);
 		strbuf_addf(&buf, "refs/heads/%s", branch_name);
-		if (!read_ref(buf.buf, &options.orig_head))
+		if (!read_ref(buf.buf, &options.orig_head)) {
+			die_if_checked_out(buf.buf, 1);
 			options.head_name = xstrdup(buf.buf);
 		/* If not is it a valid ref (branch or commit)? */
-		else if (!get_oid(branch_name, &options.orig_head))
+		} else if (!get_oid(branch_name, &options.orig_head))
 			options.head_name = NULL;
 		else
 			die(_("fatal: no such branch/commit '%s'"),

t/t3400-rebase.sh

@@ -377,4 +377,22 @@ test_expect_success 'rebase -c rebase.useBuiltin=false warning' '
 	test_must_be_empty err
 '
 
+test_expect_success 'switch to branch checked out here' '
+	git checkout master &&
+	git rebase master master
+'
+
+test_expect_success 'switch to branch not checked out' '
+	git checkout master &&
+	git branch other &&
+	git rebase master other
+'
+
+test_expect_success 'refuse to switch to branch checked out elsewhere' '
+	git checkout master &&
+	git worktree add wt &&
+	test_must_fail git -C wt rebase master master 2>err &&
+	test_i18ngrep "already checked out" err
+'
+
 test_done