Update checkout with new strategies & behavior

We're already in the git_config namespace, there is no need to repeat
it.

Win32 CryptoAPI and CNG support for SHA1

Bump the builtin http-parser to v2.0

Fix memory leaks

repository: Refine repository_head() error report

The indexer handles this better than the fetch
logic does.

create FETCH_HEAD specially instead of as a ref file

Duplicate tree entries

Topic/explicit index errors

This fixes some various warnings that showed up in Travis and
a couple uses of uninitialized memory and one memory leak.

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.

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.

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.

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.

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.

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.