Add git_oid_nfmt - a flexible OID formatter

I frequently want to the the first N digits of an OID formatted
as a string and I'd like it to be efficient.  This function makes
that easy and I could rewrite the OID formatters in terms of it.
Add git_diff_print_raw printing helper

Makes it easier to emulate the --raw option
Fix delta compare to use correct pathname

The delta cmp function needs to choose the correct path for
ordering when a delta is ADDED, RENAMED, or COPIED.
Fix issues with git_diff_find_similar

There are a number of bugs in the rename code that only were
obvious when I started testing it against large old repos with
more complex patterns.  (The code to do that testing is not ready
to merge with libgit2, but I do plan to add more thorough tests.)

This contains a significant number of changes and also tweaks the
public API slightly to make emulating core git easier.

Most notably, this separates the GIT_DIFF_FIND_AND_BREAK_REWRITES
flag into FIND_REWRITES (which adds a self-similarity score to
every modified file) and BREAK_REWRITES (which splits the modified
deltas into add/remove pairs in the diff list).  When you do a raw
output of core git, rewrites show up as M090 or such, not at A and
D output, so I wanted to be able to emulate that.

Publicly, this also changes the flags to be uint16_t since we
don't need values out of that range.

Internally, this contains significant changes from a number of
small bug fixes (like using the wrong side of the diff to decide
if the object could be found in the ODB vs the workdir) to larger
issues about which files can and should be compared and how the
various edge cases of similarity scores should be treated.

Honestly, I don't think this is the last update that will have to
be made to this code, but I think this moves us closer to correct
behavior and I tried to document the code so it would be easier
to follow..
Extend diff example

Add --raw output format and (some) options to invoke rename/copy
detection on the diff.
More git_diff_find_similar improvements

- Add new GIT_DIFF_FIND_EXACT_MATCH_ONLY flag to do similarity
  matching without using the similarity metric (i.e. only compare
  the SHA).
- Clean up the similarity measurement code to more rigorously
  distinguish between files that are not similar and files that
  are not comparable (previously, a 0 could either mean that the
  files could not be compared or that they were totally different)
- When splitting a MODIFIED file into a DELETE/ADD pair, actually
  make a DELETED/UNTRACKED pair if the right side of the diff is
  from the working directory.  This prevents an odd mix of ADDED
  and UNTRACKED files on workdir diffs.
Add more diff rename detection tests

This adds a bunch more rename detection tests including checks
vs the working directory, the new exact match options, some more
whitespace variants, etc.

This also adds a git_futils_writebuffer helper function and uses
it in checkout.  This is mainly added because I wanted an easy
way to write out a git_buf to disk inside my test code.
Significant rename detection rewrite

This flips rename detection around so instead of creating a
forward mapping from deltas to possible rename targets, instead
it creates a reverse mapping, looking at possible targets and
trying to find a source that they could have been renamed or
copied from.  This is important because each output can only
have a single source, but a given source could map to multiple
outputs (in the form of COPIED records).

Additionally, this makes a couple of tweaks to the public rename
detection APIs, mostly renaming a couple of options that control
the behavior to make more sense and to be more like core Git.

I walked through the tests looking at the exact results and
updated the expectations based on what I saw.  The new code is
different from the old because it cannot give some nonsense
results (like A was renamed to both B and C) which were part of
the outputs previously.
Fix dereference of freed delta

I was accidentally using a value that I had just freed.  This
moves the clearing of the delta internal flags into a better place.
More diff rename tests; better split swap handling

This adds a couple more tests of different rename scenarios.

Also, this fixes a problem with the case where you have two
"split" deltas and the left half of one matches the right half of
the other.  That case was already being handled, but in the wrong
order in a way that could result in bad output.  Also, if the swap
also happened to put the other two halves into the correct place
(i.e. two files exchanged places with each other), then the second
delta was left with the SPLIT flag set when it really should be
cleared.
Move core.abbrev lookup out of diff print loop

This moves the GIT_CVAR_ABBREV lookup out of the loop.  Also, this
fixes git_diff_print_raw to actually use that constant instead of
hardcoding 7 characters.
Fill out diff rename test coverage

This extends the rename tests to make sure that every rename
scenario in the inner loop of git_diff_find_similar is actually
exercised.  Also, fixes an incorrect assert that was in one of
the clauses that was not previously being exercised.
Merge pull request #1595 from arrbee/even-more-rename-fixes

Even more rename detection fixes