When looking for an object by prefix, we query all the backends so that
we can ensure that there is no ambiguity.  We need to reset the `error`
value between backends; otherwise the first backend may find an object
by prefix, but subsequent backends may not.  If we do not reset the
`error` value then it will remain at `GIT_ENOTFOUND` and `read_prefix_1`
will fail, despite having actually found an object.

Ensure packfiles with different contents have different names

Update to forced checkout and untracked files

Only ignore `EBUSY` from `rmdir` when the `GIT_RMDIR_SKIP_NONEMPTY` bit
is set.

When deleting a directory during checkout, do not simply delete the
directory, since there may be untracked files.  Instead, go into
the iterator and examine each file.

In the original code (the code with the faulty assumption), we look to
see if there's an index entry beneath the directory that we want to
remove.   Eg, it looks to see if we have a workdir entry foo and an
index entry foo/bar.txt. If this is not the case, then the working
directory must have precious files in that directory. This part is okay.
The part that's not okay is if there is an index entry foo/bar.txt. It
just blows away the whole damned directory.

That's not cool.

Instead, by simply pushing the directory itself onto the stack and
iterating each entry, we will deal with the files one by one - whether
they're in the index (and can be force removed) or not (and are
precious).

The original code was a bad optimization, assuming that we didn't need
to git_iterator_advance_into if there was any index entry in the folder.
That's wrong - we could have optimized this iff all folder entries are
in the index.

Instead, we need to simply dig into the directory and analyze its
entries.

Initially, the setting has been solely used to enable the use of
`fsync()` when creating objects. Since then, the use has been extended
to also cover references and index files. As the option is not yet part
of any release, we can still correct this by renaming the option to
something more sensible, indicating not only correlation to objects.

This commit renames the option to `GIT_OPT_ENABLE_FSYNC_GITDIR`. We also
move the variable from the object to repository source code.

Buffer growing cleanups

Coverity fixes