checkout: add advice for ambiguous "checkout <branch>"

As the "checkout" documentation describes:

    If <branch> is not found but there does exist a tracking branch in
    exactly one remote (call it <remote>) with a matching name, treat
    as equivalent to [...] <remote>/<branch.

This is a really useful feature. The problem is that when you add
another remote (e.g. a fork), git won't find a unique branch name
anymore, and will instead print this unhelpful message:

    $ git checkout master
    error: pathspec 'master' did not match any file(s) known to git

Now it will, on my git.git checkout, print:

    $ ./git --exec-path=$PWD checkout master
    error: pathspec 'master' did not match any file(s) known to git.
    hint: 'master' matched more than one remote tracking branch.
    hint: We found 26 remotes with a reference that matched. So we fell back
    hint: on trying to resolve the argument as a path, but failed there too!
    hint:
    hint: If you meant to check out a remote tracking branch on, e.g. 'origin',
    hint: you can do so by fully qualifying the name with the --track option:
    hint:
    hint:     git checkout --track origin/<name>

Note that the "error: pathspec[...]" message is still printed. This is
because whatever else checkout may have tried earlier, its final
fallback is to try to resolve the argument as a path. E.g. in this
case:

    $ ./git --exec-path=$PWD checkout master pu
    error: pathspec 'master' did not match any file(s) known to git.
    error: pathspec 'pu' did not match any file(s) known to git.

There we don't print the "hint:" implicitly due to earlier logic
around the DWIM fallback. That fallback is only used if it looks like
we have one argument that might be a branch.

I can't think of an intrinsic reason for why we couldn't in some
future change skip printing the "error: pathspec[...]" error. However,
to do so we'd need to pass something down to checkout_paths() to make
it suppress printing an error on its own, and for us to be confident
that we're not silencing cases where those errors are meaningful.

I don't think that's worth it since determining whether that's the
case could easily change due to future changes in the checkout logic.

Signed-off-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 2 insertions, 0 deletions

diff --git a/advice.c b/advice.c
index 370a56d..75e7ded 100644
--- a/advice.c
+++ b/advice.c
@@ -21,6 +21,7 @@ int advice_add_embedded_repo = 1;
 int advice_ignored_hook = 1;
 int advice_waiting_for_editor = 1;
 int advice_graft_file_deprecated = 1;
+int advice_checkout_ambiguous_remote_branch_name = 1;
 
 static int advice_use_color = -1;
 static char advice_colors[][COLOR_MAXLEN] = {
@@ -72,6 +73,7 @@ static struct {
 	{ "ignoredhook", &advice_ignored_hook },
 	{ "waitingforeditor", &advice_waiting_for_editor },
 	{ "graftfiledeprecated", &advice_graft_file_deprecated },
+	{ "checkoutambiguousremotebranchname", &advice_checkout_ambiguous_remote_branch_name },
 
 	/* make this an alias for backward compatibility */
 	{ "pushnonfastforward", &advice_push_update_rejected }