checkout: finish switch when tracking config cannot be written

When the tracking config cannot be written (for example, because .git/config.lock exists), git switch -c and checkout -b stop halfway. The worktree, the index and the new branch ref are already updated, but HEAD stays on the old branch, and the changes are left staged.

* 1/2 makes setup_tracking() return an error instead of calling exit(). No behavior change yet.
* 2/2 makes switch and checkout finish the switch and exit with status 0, as push -u, clone and fetch already do. git branch still fails.

These patches were first sent as part of v2 of a series that also fixed the hint
(https://lore.kernel.org/git/[email protected]/).
Junio asked whether they can be treated separately from the hint fix
(https://lore.kernel.org/git/[email protected]/), and I replied that they can
(https://lore.kernel.org/git/CAP8tz2KOXHLjc5V1o1tZZWSsgOjVrp-ekH7H5KszBGK+hRqwrg@mail.gmail.com/).
The direction was agreed in the bug report thread
(https://lore.kernel.org/git/[email protected]/).

Making the switch fully atomic, so that a failure undoes the worktree, the index, the ref and the config, is left for a separate topic.

tsukasa inoue (2):
branch: return an error from setup_tracking()
checkout: finish switching when the tracking config cannot be written

 branch.c                  | 35 +++++++++++++++++++++--------------
 branch.h                  | 21 +++++++++++++--------
 builtin/branch.c          | 17 ++++++++++++-----
 builtin/checkout.c        |  6 ++++++
 t/t2027-checkout-track.sh | 20 ++++++++++++++++++++
 t/t2060-switch.sh         | 20 ++++++++++++++++++++
 t/t3200-branch.sh         | 13 +++++++++++++
 7 files changed, 105 insertions(+), 27 deletions(-)

base-commit: 47fdf2208c5b94c8cc01c106b8f7c85efb91c926
Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-2275%2FtsukasaI%2Ffix%2Fcheckout-tracking-config-failure-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-2275/tsukasaI/fix/checkout-tracking-config-failure-v1
Pull-Request: https://github.com/gitgitgadget/git/pull/2275

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