branch: deprecate --set-upstream and show help if we detect possible mistaken use

This interface is error prone, and a better one (--set-upstream-to)
exists. Add a message listing the alternatives and suggest how to fix
a --set-upstream invocation in case the user only gives one argument
which causes a local branch with the same name as a remote-tracking
one to be created. The typical case is

    git branch --set-upstream origin/master

when the user meant

    git branch --set-upstream master origin/master

assuming that the current branch is master. Show a message telling the
user how to undo their action and get what they wanted. For the
command above, the message would be

The --set-upstream flag is deprecated and will be removed. Consider using --track or --set-upstream-to
Branch origin/master set up to track local branch master.

If you wanted to make 'master' track 'origin/master', do this:

    git branch -d origin/master
    git branch --set-upstream-to origin/master
Signed-off-by: NCarlos Martín Nieto <cmn@elego.de>
Signed-off-by: NJunio C Hamano <gitster@pobox.com>

builtin/branch.c

@@ -881,10 +881,36 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		git_config_set_multivar(buf.buf, NULL, NULL, 1);
 		strbuf_release(&buf);
 	} else if (argc > 0 && argc <= 2) {
+		struct branch *branch = branch_get(argv[0]);
+		int branch_existed = 0, remote_tracking = 0;
+		struct strbuf buf = STRBUF_INIT;
+
 		if (kinds != REF_LOCAL_BRANCH)
 			die(_("-a and -r options to 'git branch' do not make sense with a branch name"));
+
+		if (track == BRANCH_TRACK_OVERRIDE)
+			fprintf(stderr, _("The --set-upstream flag is deprecated and will be removed. Consider using --track or --set-upstream-to\n"));
+
+		strbuf_addf(&buf, "refs/remotes/%s", branch->name);
+		remote_tracking = ref_exists(buf.buf);
+		strbuf_release(&buf);
+
+		branch_existed = ref_exists(branch->refname);
 		create_branch(head, argv[0], (argc == 2) ? argv[1] : head,
 			      force_create, reflog, 0, quiet, track);
+
+		/*
+		 * We only show the instructions if the user gave us
+		 * one branch which doesn't exist locally, but is the
+		 * name of a remote-tracking branch.
+		 */
+		if (argc == 1 && track == BRANCH_TRACK_OVERRIDE &&
+		    !branch_existed && remote_tracking) {
+			fprintf(stderr, _("\nIf you wanted to make '%s' track '%s', do this:\n\n"), head, branch->name);
+			fprintf(stderr, _("    git branch -d %s\n"), branch->name);
+			fprintf(stderr, _("    git branch --set-upstream-to %s\n"), branch->name);
+		}
+
 	} else
 		usage_with_options(builtin_branch_usage, options);
 

t/t3200-branch.sh

@@ -405,6 +405,40 @@ test_expect_success 'test --unset-upstream on a particular branch' \
      test_must_fail git config branch.my14.remote &&
      test_must_fail git config branch.my14.merge'
 
+test_expect_success '--set-upstream shows message when creating a new branch that exists as remote-tracking' \
+    'git update-ref refs/remotes/origin/master HEAD &&
+     git branch --set-upstream origin/master 2>actual &&
+     test_when_finished git update-ref -d refs/remotes/origin/master &&
+     test_when_finished git branch -d origin/master &&
+     cat >expected <<EOF &&
+The --set-upstream flag is deprecated and will be removed. Consider using --track or --set-upstream-to
+
+If you wanted to make '"'master'"' track '"'origin/master'"', do this:
+
+    git branch -d origin/master
+    git branch --set-upstream-to origin/master
+EOF
+     test_cmp expected actual
+'
+
+test_expect_success '--set-upstream with two args only shows the deprecation message' \
+    'git branch --set-upstream master my13 2>actual &&
+     test_when_finished git branch --unset-upstream master &&
+     cat >expected <<EOF &&
+The --set-upstream flag is deprecated and will be removed. Consider using --track or --set-upstream-to
+EOF
+     test_cmp expected actual
+'
+
+test_expect_success '--set-upstream with one arg only shows the deprecation message if the branch existed' \
+    'git branch --set-upstream my13 2>actual &&
+     test_when_finished git branch --unset-upstream my13 &&
+     cat >expected <<EOF &&
+The --set-upstream flag is deprecated and will be removed. Consider using --track or --set-upstream-to
+EOF
+     test_cmp expected actual
+'
+
 # Keep this test last, as it changes the current branch
 cat >expect <<EOF
 $_z40 $HEAD $GIT_COMMITTER_NAME <$GIT_COMMITTER_EMAIL> 1117150200 +0000	branch: Created from master