Make iterators use GIT_ITEROVER & smart advance

1. internal iterators now return GIT_ITEROVER when you go past the
   last item in the iteration.
2. git_iterator_advance will "advance" to the first item in the
   iteration if it is called immediately after creating the
   iterator, which allows a simpler idiom for basic iteration.
3. if git_iterator_advance encounters an error reading data (e.g.
   a missing tree or an unreadable file), it returns the error
   but also attempts to advance past the invalid data to prevent
   an infinite loop.

Updated all tests and internal usage of iterators to account for
these new behaviors.

packbuilder: also write index in git_packbuilder_write

git_packbuilder_write() used to write a packfile to the passed file
path. Instead, ask for a destination directory and create both the
packfile and an index, as most users probably do expect.

tests: don't verify SSH unsupported with GIT_SSH

Fix documentation of git_branch_delete.

The reference should be freed by the user, not the library.

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.