Extensions to rmdir and mkdir utilities

* Rework GIT_DIRREMOVAL values to GIT_RMDIR flags, allowing
  combinations of flags
* Add GIT_RMDIR_EMPTY_PARENTS flag to remove parent dirs that
  are left empty after removal
* Add GIT_MKDIR_VERIFY_DIR to give an error if item is a file,
  not a dir (previously an EEXISTS error was ignored, even for
  files) and enable this flag for git_futils_mkpath2file call
* Improve accuracy of error messages from git_futils_mkdir
Fix checkout behavior when its hands are tied

So, @nulltoken created a failing test case for checkout that
proved to be particularly daunting.  If checkout is given only
a very limited strategy mask (e.g. just GIT_CHECKOUT_CREATE_MISSING)
then it is possible for typechange/rename modifications to leave it
unable to complete the request.  That's okay, but the existing code
did not have enough information not to generate an error (at least
for tree/blob conflicts).

This led me to a significant reorganization of the code to handle
the failing case, but it has three benefits:

1. The test case is handled correctly (I think)
2. The new code should actually be much faster than the old code
   since I decided to make checkout aware of diff list internals.
3. The progress value accuracy is hugely increased since I added
   a fourth pass which calculates exactly what work needs to be
   done before doing anything.
Clean up a couple things missed in rebase
Make iterator ignore eval lazy

This makes it so that the check if a file is ignored will be
deferred until requested on the workdir iterator, instead of
aggressively evaluating the ignore rules for each entry.  This
should improve performance because there will be no need to
check ignore rules for files that are already in the index.
Move pathspec code in separate files

Diff uses a `git_strarray` of path specs to represent a subset
of all files to be processed.  It is useful to be able to reuse
this filtering in other places outside diff, so I've moved it
into a standalone set of utilities.
Some diff refactorings to help code reuse

There are some diff functions that are useful in a rewritten
checkout and this lays some groundwork for that.  This contains
three main things:

1. Share the function diff uses to calculate the OID for a file
   in the working directory (now named `git_diff__oid_for_file`
2. Add a `git_diff__paired_foreach` function to iterator over
   two diff lists concurrently.  Convert status to use it.
3. Move all the string/prefix/index entry comparisons into
   function pointers inside the `git_diff_list` object so they
   can be switched between case sensitive and insensitive
   versions.  This makes them easier to reuse in various
   functions without replicating logic.  As part of this, move
   a couple of index functions out of diff.c and into index.c.
Rework checkout with new strategy options

This is a major reworking of checkout strategy options.  The
checkout code is now sensitive to the contents of the HEAD tree
and the new options allow you to update the working tree so that
it will match the index content only when it previously matched
the contents of the HEAD.  This allows you to, for example, to
distinguish between removing files that are in the HEAD but not
in the index, vs just removing all untracked files.

Because of various corner cases that arise, etc., this required
some additional capabilities in rmdir and other utility functions.

This includes the beginnings of an implementation of code to read
a partial tree into the index based on a pathspec, but that is
not enabled because of the possibility of creating conflicting
index entries.
fix regression in diff with submodule oid
Update reset hard test

The `git_reset` API with the HARD option is still slightly
broken, but this test now does exercise the ability of the
command to revert modified files.
Fix various cross-platform build issues

This fixes a number of warnings and problems with cross-platform
builds.  Among other things, it's not safe to name a member of a
structure "strcmp" because that may be #defined.
Fix warnings and valgrind issues

This fixes some various warnings that showed up in Travis and
a couple uses of uninitialized memory and one memory leak.
Merge pull request #1016 from arrbee/fix-checkout-dir-removal

Update checkout with new strategies & behavior