[PATCH 2/3] branch: suggest how to undo a --set-upstream when given one branch

["Carlos Martín Nieto" <cmn@elego.de>]

This interface is error prone, and a better one (--set-upstream-to)
exists. Suggest how to fix a --set-upstream invocation in case the
user only gives one argument, which makes it likely that he meant to
do the opposite, like with

    git branch --set-upstream origin/master

when they meant one of

    git branch --set-upstream origin/master master
    git branch --set-upstream-to origin/master

Signed-off-by: Carlos Martín Nieto <cmn@elego.de>
---
 builtin/branch.c |   22 ++++++++++++++++++++++
 1 file changed, 22 insertions(+)

diff --git a/builtin/branch.c b/builtin/branch.c
index c886fc0..5551227 100644
--- a/builtin/branch.c
+++ b/builtin/branch.c
@@ -864,10 +864,32 @@ int cmd_branch(int argc, const char **argv, const char *prefix)
 		   info and making sure new_upstream is correct */
 		create_branch(head, branch->name, new_upstream, 0, 0, 0, quiet, BRANCH_TRACK_OVERRIDE);
 	} else if (argc > 0 && argc <= 2) {
+		struct branch *branch = branch_get(argv[0]);
+		const char *old_upstream = NULL;
+		int branch_existed = 0;
+
 		if (kinds != REF_LOCAL_BRANCH)
 			die(_("-a and -r options to 'git branch' do not make sense with a branch name"));
+
+		/* Save what argv[0] was pointing to so we can give
+		   the --set-upstream-to hint */
+		if (branch_has_merge_config(branch))
+		  old_upstream = shorten_unambiguous_ref(branch->merge[0]->dst, 0);
+
+		branch_existed = ref_exists(branch->refname);
 		create_branch(head, argv[0], (argc == 2) ? argv[1] : head,
 			      force_create, reflog, 0, quiet, track);
+
+		if (argc == 1) {
+			printf("If you wanted to make '%s' track '%s', do this:\n", head, argv[0]);
+			if (branch_existed)
+				printf(" $ git branch --set-upstream '%s' '%s'\n", argv[0], old_upstream);
+			else
+				printf(" $ git branch -d '%s'\n", argv[0]);
+
+			printf(" $ git branch --set-upstream-to '%s'\n", argv[0]);
+		}
+
 	} else
 		usage_with_options(builtin_branch_usage, options);
 
-- 
1.7.10.2.1.g8c77c3c