The current code issues a lot of strncmp() calls in order to check for
the end of the header, simply in order to copy it and start going
through it again. These are a lot of calls for something we can check as
we go along. Knowing the amount of parents beforehand to reduce
allocations in extreme cases does not make up for them.

Instead start parsing immediately and check for the double-newline after
each header field, leaving the raw_header allocation for the end, which
lets us go through the header once and reduces the amount of strncmp()
calls significantly.

In unscientific testing, this has reduced a shortlog-like usage (walking
though the whole history of a branch and extracting data from the
commits) of git.git from ~830ms to ~700ms and makes the time we spend in
strncmp() negligible.

Corrections for merge working directory validation

Validating the workdir should not compare HEAD to working
directory - this is both inefficient (as it ignores the cache)
and incorrect.  If we had legitimately allowed changes in the
index (identical to the merge result) then comparing HEAD to
workdir would reject these changes as different.  Further, this
will identify files that were filtered strangely as modified,
while testing with the cache would prevent this.

Also, it's stupid slow.

Correct "new" id for reattached-HEAD reflog entry

Add `git_reference_is_note`.

Update to latest clar

Reflog completionism

Index tree-bob collision

If you are checking out NONE, then don't remove.

The checkout code used to defer removal of "blocking" files in
checkouts until the blocked item was actually being written (since
we have already checked that the removing the block is acceptable
according to the update rules).  Unfortunately, this resulted in
an intermediate index state where both the blocking and new items
were in the index which is no longer allowed.  Now we just remove
the blocking item in the first pass so it never needs to coexist.

In cases where there are typechanges, this could result in a bit
more churn of removing and recreating intermediate directories,
but I'm going to assume that is an unusual case and the churn will
not be too costly.

Fix errors with zstream (and a variety of other cleanups)

And don't terminate if there isn't space for it (since it's binary
data, it's not worth a reallocation).

There were some confusing issues mixing up the number of bytes
written to the zstream output buffer with the number of bytes
consumed from the zstream input.  This reorganizes the zstream
API and makes it easier to deflate an arbitrarily large input
while still using a fixed size output.