The trace logging callbacks should match the other callback naming
conventions, using the `_cb` suffix instead of a `_callback` suffix.

The credential callbacks should match the other callback naming
conventions, using the `_cb` suffix instead of a `_callback` suffix.

ignore: handle escaped trailing whitespace

Ignore: only treat one leading slash as a root identifier

The gitignore's pattern format specifies that "Trailing spaces
are ignored unless they are quoted with backslash ("\")". We do
not honor this currently and will treat a pattern "foo\ " as if
it was "foo\" only and a pattern "foo\ \ " as "foo\ \".

Fix our code to handle those special cases and add tests to avoid
regressions.

The stripping of trailing spaces currently happens as part of
`git_attr_fnmatch__parse`. As we aren't currently parsing
trailing whitespaces correct in case they're escaped, we'll have
to change that code, though. To make actual behavioural change
easier to review, refactor the code up-front by pulling it out
into its own function that is expected to retain the exact same
functionality as before. Like this, the fix will be trivial to
apply.

When `allow_space` is unset, ensure that leading whitespace is not
skipped.

When evicting cache entries, we first retrieve the object that is
to be evicted, delete the object and then finally delete the key
from the cache. In case where the cache eviction caused us to
free the cached object, though, its key will point to invalid
memory now when trying to remove it from the cache map. On my
system, this causes us to not properly remove the key from the
map, as its memory has been overwritten already and thus the key
lookup it will fail and we cannot delete it.

Fix this by only decrementing the refcount of the evictee after
we have removed it from our cache map. Add a test that caused a
segfault previous to that change.

See: https://www.netbsd.org/changes/changes-7.0.html

error was never initialized and a garbage value returned on success.