[RFC] git clean: add persistent exclude patterns

Hi,

I would like to propose a way to configure persistent exclusion
patterns for `git clean`.

Currently, `git clean` supports:

-e <pattern>
--exclude=<pattern>

and these exclusions are deliberately still honored when `-x` is used.

For example:

git clean -fdx -e '*nogitclean*'

will remove ignored files while preserving paths matching `*nogitclean*`.

What seems to be missing is a way to make such exclusions persistent.
A possible configuration could be:

[clean]
    exclude = *nogitclean*

or, for multiple patterns:

[clean]
    exclude = *nogitclean*
    exclude = important-local-data/

which could be configured globally or per repository:

git config --global --add clean.exclude '*nogitclean*'

The intended semantics would be the same as if the configured patterns
had been supplied using `git clean -e`: in particular, they would
continue to protect matching paths when `git clean -x` is used.

The use case is slightly different from `.gitignore` or
`core.excludesFile`. Those answer the question "which files should Git
normally ignore?", whereas this configuration would answer "which
untracked paths should `git clean` never remove by default, even when
cleaning ignored files?"

One concrete use case is keeping local metadata or other irreplaceable
directories inside a working tree while still being able to use `git
clean -fdx` for build products. Such directories may intentionally be
ignored by Git, but `-x` currently removes them unless the
corresponding `-e` option is remembered every time.

An alternative design could be a dedicated file such as
`clean.excludesFile`, but a multi-valued `clean.exclude` setting seems
simpler and maps directly to the existing `-e` option.

Would such a configuration be considered useful and consistent with
the intended semantics of `git clean`?

Thanks,
Nicolas