Parse diff files into a `git_diff` structure.

Now that `git_diff_delta` data can be produced by reading patch
file data, which may have an abbreviated oid, allow consumers to
know that the id is abbreviated.

When determining diffs between two iterators we may need to
recurse into an unmatched directory for the "new" iterator when
it is either a prefix to the current item of the "old" iterator
or when untracked/ignored changes are requested by the user and
the directory is untracked/ignored.

When advancing into the directory and no files are found, we will
get back `GIT_ENOTFOUND`. If so, we simply skip the directory,
handling resulting unmatched old items in the next iteration. The
other case of `iterator_advance_into` returning either
`GIT_NOERROR` or any other error but `GIT_ENOTFOUND` will be
handled by the caller, which will now either compare the first
directory entry of the "new" iterator in case of `GIT_ENOERROR`
or abort on other cases.

Improve readability of the code to make the above logic more
clear.

Remove some unused functions, refactor some ugliness.

When a directory is removed out from underneath us, stop trying to
manipulate it.

Drop some of the layers of indirection between the workdir and the
filesystem iterators.  This makes the code a little bit easier to
follow, and reduces the number of unnecessary allocations a bit as
well.  (Prior to this, when we filter entries, we would allocate them,
filter them and then free them; now we do the filtering before
allocation.)

Also, rename `git_iterator_advance_over_with_status` to just
`git_iterator_advance_over`.  Mostly because it's a fucking long-ass
function name otherwise.

When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.

When examining the working directory and determining whether it's
up-to-date, only consider the nanoseconds in the index entry when
built with `GIT_USE_NSEC`.  This prevents us from believing that
the working directory is always dirty when the index was originally
written with a git client that uinderstands nsecs (like git 2.x).