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>

Documentation/config.txt

@@ -344,6 +344,13 @@ advice.*::
 		Advice shown when you used linkgit:git-checkout[1] to
 		move to the detach HEAD state, to instruct how to create
 		a local branch after the fact.
+	checkoutAmbiguousRemoteBranchName::
+		Advice shown when the argument to
+		linkgit:git-checkout[1] ambiguously resolves to a
+		remote tracking branch on more than one remote in
+		situations where an unambiguous argument would have
+		otherwise caused a remote-tracking branch to be
+		checked out.
 	amWorkDir::
 		Advice that shows the location of the patch file when
 		linkgit:git-am[1] fails to apply it.

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 }

advice.h

@@ -22,6 +22,7 @@ extern int advice_add_embedded_repo;
 extern int advice_ignored_hook;
 extern int advice_waiting_for_editor;
 extern int advice_graft_file_deprecated;
+extern int advice_checkout_ambiguous_remote_branch_name;
 
 int git_default_advice_config(const char *var, const char *value);
 __attribute__((format (printf, 1, 2)))

builtin/checkout.c

@@ -22,6 +22,7 @@
 #include "resolve-undo.h"
 #include "submodule-config.h"
 #include "submodule.h"
+#include "advice.h"
 
 static const char * const checkout_usage[] = {
 	N_("git checkout [<options>] <branch>"),
@@ -1267,6 +1268,18 @@ int cmd_checkout(int argc, const char **argv, const char *prefix)
 	UNLEAK(opts);
 	if (opts.patch_mode || opts.pathspec.nr) {
 		int ret = checkout_paths(&opts, new_branch_info.name);
+		if (ret && dwim_remotes_matched > 1 &&
+		    advice_checkout_ambiguous_remote_branch_name)
+			advise(_("'%s' matched more than one remote tracking branch.\n"
+				 "We found %d remotes with a reference that matched. So we fell back\n"
+				 "on trying to resolve the argument as a path, but failed there too!\n"
+				 "\n"
+				 "If you meant to check out a remote tracking branch on, e.g. 'origin',\n"
+				 "you can do so by fully qualifying the name with the --track option:\n"
+				 "\n"
+				 "    git checkout --track origin/<name>"),
+			       argv[0],
+			       dwim_remotes_matched);
 		return ret;
 	} else {
 		return checkout_branch(&opts, &new_branch_info);

t/t2024-checkout-dwim.sh

@@ -76,6 +76,20 @@ test_expect_success 'checkout of branch from multiple remotes fails #1' '
 	test_branch master
 '
 
+test_expect_success 'checkout of branch from multiple remotes fails with advice' '
+	git checkout -B master &&
+	test_might_fail git branch -D foo &&
+	test_must_fail git checkout foo 2>stderr &&
+	test_branch master &&
+	status_uno_is_clean &&
+	test_i18ngrep "^hint: " stderr &&
+	test_must_fail git -c advice.checkoutAmbiguousRemoteBranchName=false \
+		checkout foo 2>stderr &&
+	test_branch master &&
+	status_uno_is_clean &&
+	test_i18ngrep ! "^hint: " stderr
+'
+
 test_expect_success 'checkout of branch from a single remote succeeds #1' '
 	git checkout -B master &&
 	test_might_fail git branch -D bar &&