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.

Currently, the reflog disallows any entries that have a message with
newlines, as that would effectively break the reflog format, which may
contain a single line per entry, only. Upstream git behaves a bit
differently, though, especially when considering stashes: instead of
rejecting any reflog entry with newlines, git will simply replace
newlines with spaces. E.g. executing 'git stash push -m "foo\nbar"' will
create a reflog entry with "foo bar" as entry message.

This commit adjusts our own logic to stop rejecting commit messages with
newlines. Previously, this logic was part of `git_reflog_append`, only.
There is a second place though where we add reflog entries, which is the
serialization code in the filesystem refdb. As it didn't contain any
sanity checks whatsoever, the refdb would have been perfectly happy to
write malformatted reflog entries to the disk. This is being fixed with
the same logic as for the reflog itself.

The flag GIT_FILEBUF_FORCE currently does two things:
     1. It will cause the filebuf to create non-existing leading
        directories for the file that is about to be written.
     2. It will forcibly remove any pre-existing locks.
While most call sites actually do want (1), they do not want to
remove pre-existing locks, as that renders the locking mechanisms
effectively useless.
Introduce a new flag `GIT_FILEBUF_CREATE_LEADING_DIRS` to
separate both behaviours cleanly from each other and convert
callers to use it instead of `GIT_FILEBUF_FORCE` to have them
honor locked files correctly.

As this conversion removes all current users of `GIT_FILEBUF_FORCE`,
this commit removes the flag altogether.

This fixes part of the issue where, given a concurrent `git pack-refs`,
a ref lookup could return an old, vestigial value from the packed file,
as the valid loose one would have been deleted.