untracked-cache: support '--untracked-files=all' if configured

untracked-cache: support '--untracked-files=all' if configured

Untracked cache was originally designed to only work with
"--untracked-files=normal", and is bypassed when
"--untracked-files=all" is requested, but this causes performance
issues for UI tooling that wants to see "all" on a frequent basis.

On the other hand, the conditions that altogether prevented
applicability to the "all" mode no longer seem to apply, after
several major refactors in recent years; this possibility was
discussed in
81153d02-8e7a-be59-e709-e90cd5906f3a@jeffhostetler.com and
CABPp-BFiwzzUgiTj_zu+vF5x20L0=1cf25cHwk7KZQj2YkVzXw@mail.gmail.com,
and somewhat confirmed experimentally by several users using a
version of this patch to use untracked cache with -uall for about a
year.

When 'git status' runs without using the untracked cache, on a large
repo, on windows, with fsmonitor, it can run very slowly. This can
make GUIs that need to use "-uall" (and therefore currently bypass
untracked cache) unusable when fsmonitor is enabled, on such large
repos.

To partially address this, align the supported directory flags for the
stored untracked cache data with the git config. If a user specifies
an '--untracked-files=' commandline parameter that does not align with
their 'status.showuntrackedfiles' config value, then the untracked
cache will be ignored - as it is for other unsupported situations like
when a pathspec is specified.

If the previously stored flags no longer match the current
configuration, but the currently-applicable flags do match the current
configuration, then discard the previously stored untracked cache
data.

For most users there will be no change in behavior. Users who need
'--untracked-files=all' to perform well will now have the option of
setting "status.showuntrackedfiles" to "all" for better / more
consistent performance.

Users who need '--untracked-files=all' to perform well for their
tooling AND prefer to avoid the verbosity of "all" when running
git status explicitly without options... are out of luck for now (no
change).

Users who have the "status.showuntrackedfiles" config set to "all"
and yet frequently explicitly call
'git status --untracked-files=normal' (and use the untracked cache)
are the only ones who will be disadvantaged by this change. Their
"--untracked-files=normal" calls will, after this change, no longer
use the untracked cache.

Signed-off-by: Tao Klerks <tao@klerks.biz>
Signed-off-by: Junio C Hamano <gitster@pobox.com>