completion: improve handling of DWIM mode for switch/checkout

A new helper, __git_find_last_on_cmdline is introduced, similar to the
already existing __git_find_on_cmdline, but which operates in reverse,
finding the *last* matching word of the provided wordlist.

Use this in a new __git_checkout_default_dwim_mode() function that will
determine when to enable listing of DWIM remote branches.

The __git_find_last_on_cmdline() function is used to determine which
--guess or --no-guess is in effect. If either one is provided, then we
unconditionally enable or disable the DWIM mode based on the last
provided option.

If neither --guess nor --no-guess is provided, then we check for
--no-track, and finally for GIT_COMPLETION_CHECKOUT_NO_GUESS=1.

This function is then used in _git_switch and _git_checkout to improve
the handling for when we enable listing of these DWIM remote branches.

This new logic is more robust, as we will correctly identify superseded
options, and ensure that both _git_switch and _git_checkout enable DWIM
in similar ways.

We can now update a few tests to indicate they pass. A few of the tests
previously added to highlight issues with the old DWIM logic still fail.
This is because of a separate issue related to the default completion
behavior of git switch, which will be addressed in a future change.

Additionally, due to this change, a few tests for the -b/-B handling of
git checkout now fail. This is a minor regression, and will be fixed by
a following change that improves the overall handling of -b/-B. Mark
these tests as known breakages for now.

Signed-off-by: Jacob Keller <jacob.keller@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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