qsort_r appeared in glibc 2.8

Various cleanups and tweaks

This adds ~/ prefix expansion for the value of core.attributesfile
and core.excludesfile, plus it fixes the fact that the attributes
cache was holding on to the string data from the config for a long
time (instead of making its own strdup) which could have caused a
problem if the config was refreshed.  Adds a test for the new
expansion capability.

This improves the docs for GIT_DIFF_INCLUDE_UNTRACKED_CONTENT as
well as the other flags related to UNTRACKED items in diff, plus
it makes that flag now automatically turn on
GIT_DIFF_INCLUDE_UNTRACKED which seems like a reasonable dwim type
of change.

The GIT_CONFIG_LEVEL constants actually work well as an enum
because they are mutually exclusive, so this adds a typedef to
the enum and uses that everywhere that one of these constants are
expected, instead of the old code that typically used an unsigned
int.

This adds docs for the cache control options to git_libgit2_opts
and also tweaks the cache code so that if the cache is disabled,
then the next time we attempt to insert something into the cache
in question, we will actually clear any old cached objects.

Apparently this makes things easier to bind in some languages.

qsort_r is broken on HURD, avoid

Cherry picking REUC fixes

In theory, if there was a problem reading the REUC data, the
read_reuc() routine could have left uninitialized and invalid
data in the git_index vector.  This moves the line that inserts a
new entry into the vector down to the bottom of the routine so we
know all the content is already valid.  Also, per @linquize, this
uses calloc to ensure no uninitialized data.

introduce git_conflict_iterator

merge setup

Shallow-clone detection

Even more rename detection fixes

Extend checking for config with no files

Plus a bit of extra paranoia to ensure config object has valid
contents.

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.

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.

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.

I was accidentally using a value that I had just freed.  This
moves the clearing of the delta internal flags into a better place.

Update windows resources to match the dll name

Bugfix: Return NULL in push_leaf, when trie is full

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.

os->full was set 1, but the overflowed idx_leaf was still used to index
into os->nodes a little later. Returning NULL fixes that.

Make InternalName and OriginalFilename resources reflect the name of the compiled binary.

tag: Introduce git_tag_annotation_create()

define "long name" in git_reference_name_to_id

Initialize loose_deleted before it is used for a check later on.

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.

- 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.