Introduce `git__getenv` which is a UTF-8 aware `getenv` everywhere.
Make `cl_getenv` use this to keep consistent memory handling around
return values (free everywhere, as opposed to only some platforms).
Don't use the filter's free callback to free the actual data structure
holding the filter, as we may not always actually initialize it (the
test may be skipped).
Allow custom filters with wildcard attributes, so that clients
can support some random `filter=foo` in a .gitattributes and look
up the corresponding smudge/clean commands in the configuration file.
Introduce GITTEST_INVASIVE_FS_STRUCTURE for things that are invasive
to your filesystem structure (like creating folders at your filesystem
root) and GITTEST_INVASIVE_FS_SIZE for things that write lots of data.
If you enabled core.safecrlf on an LF-ending platform, we would
error even for files with all LFs. We should only be warning on
irreversible mappings, I think.
Diff and status do not want core.safecrlf to actually raise an
error regardless of the setting, so this extends the filter API
with an additional options flags parameter and adds a flag so that
filters can be applied with GIT_FILTER_OPT_ALLOW_UNSAFE, indicating
that unsafe filter application should be downgraded from a failure
to a warning.
