branch: add flags and config to inherit tracking

It can be helpful when creating a new branch to use the existing
tracking configuration from the branch point. However, there is
currently not a method to automatically do so.

Teach git-{branch,checkout,switch} an "inherit" argument to the
"--track" option. When this is set, creating a new branch will cause the
tracking configuration to default to the configuration of the branch
point, if set.

For example, if branch "main" tracks "origin/main", and we run
`git checkout --track=inherit -b feature main`, then branch "feature"
will track "origin/main". Thus, `git status` will show us how far
ahead/behind we are from origin, and `git pull` will pull from origin.

This is particularly useful when creating branches across many
submodules, such as with `git submodule foreach ...` (or if running with
a patch such as [1], which we use at $job), as it avoids having to
manually set tracking info for each submodule.

Since we've added an argument to "--track", also add "--track=direct" as
another way to explicitly get the original "--track" behavior ("--track"
without an argument still works as well).

Finally, teach branch.autoSetupMerge a new "inherit" option. When this
is set, "--track=inherit" becomes the default behavior.

[1]: https://lore.kernel.org/git/20180927221603.148025-1-sbeller@google.com/

Signed-off-by: Josh Steadmon <steadmon@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

1 files changed, 23 insertions, 0 deletions

diff --git a/t/t2027-checkout-track.sh b/t/t2027-checkout-track.sh
index 4453741b96..dca35aa3e3 100755
--- a/t/t2027-checkout-track.sh
+++ b/t/t2027-checkout-track.sh
@@ -24,4 +24,27 @@ test_expect_success 'checkout --track -b rejects an extra path argument' '
 	test_i18ngrep "cannot be used with updating paths" err
 '
 
+test_expect_success 'checkout --track -b overrides autoSetupMerge=inherit' '
+	# Set up tracking config on main
+	test_config branch.main.remote origin &&
+	test_config branch.main.merge refs/heads/some-branch &&
+	test_config branch.autoSetupMerge inherit &&
+	# With --track=inherit, we copy the tracking config from main
+	git checkout --track=inherit -b b1 main &&
+	test_cmp_config origin branch.b1.remote &&
+	test_cmp_config refs/heads/some-branch branch.b1.merge &&
+	# With branch.autoSetupMerge=inherit, we do the same
+	git checkout -b b2 main &&
+	test_cmp_config origin branch.b2.remote &&
+	test_cmp_config refs/heads/some-branch branch.b2.merge &&
+	# But --track overrides this
+	git checkout --track -b b3 main &&
+	test_cmp_config . branch.b3.remote &&
+	test_cmp_config refs/heads/main branch.b3.merge &&
+	# And --track=direct does as well
+	git checkout --track=direct -b b4 main &&
+	test_cmp_config . branch.b4.remote &&
+	test_cmp_config refs/heads/main branch.b4.merge
+'
+
 test_done