To prevent a race condition, these wlock/rlock calls should probably be checked
for errors.

The logic to determine whether a reflog entry should be for the HEAD
reference is non-trivial. Currently, the only user of this is the
filesystem-based refdb, but with the advent of the reftable refdb we're
going to add a second user that's interested in having the same
behaviour.

Let's pull out a new function that checks whether a given reference
should cause a entry to be written to the HEAD reflog as a preparatory
step.

The logic to determine whether a reflog should be written is
non-trivial. Currently, the only user of this is the filesystem-based
refdb, but with the advent of the reftable refdb we're going to add a
second user that's interested in having the same behaviour.

Let's pull out a new function that checks whether a given reference
should cause a reflog to be written as a preparatory step.

Propagate failures caused by pool initialization errors.

The "refdb_fs.h" header contains a single struct `git_refcache` that is
not used anywhere. As a result, we can just delete the header altogether
as it doesn't have any purpose and may confuse readers.

While the `git_refdb_backend()` struct has a version, we do not
initialize it correctly when calling `git_refdb_backend_fs()`. Fix this
by adding the call to `git_refdb_init_backend()`.

As git_reference__name will reallocate storage to account for longer
names (it's actually allocator-dependent), it will cause all existing
pointers to the old object to become dangling, as they now point to
freed memory.

Fix the issue by renaming to a more descriptive name, and pass a pointer
to the actual reference that can safely be invalidated if the realloc
succeeds.

In previous versions, libgit2 could be coerced into writing reflog
messages with embedded newlines into the reflog by using
`git_stash_save` with a message containing newlines. While the root
cause is fixed now, it was noticed that upstream git is in fact able to
read such corrupted reflog messages just fine.

Make the reflog parser more lenient in order to just skip over
malformatted reflog lines to bring us in line with git. This requires us
to change an existing test that verified that we do indeed _fail_ to
parse such logs.

The refdb_fs code to parse the reflog currently uses a hand-rolled
parser. Convert it to use our `git_parse_ctx` structure instead.