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.

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.

* 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

Fixes for two segfaults

config: make git_config_open_level() work with an empty config

Valgrind reports Invalid Read when the configuration file contains some empty line

Update documentation in remote.h

Basic authentication for http and winhttp

- Update 'tests-clar/resources/config/config11' in order to reproduce the invalidread with the unittest (just added some \n at the end of the file)
- Fix config_file.c

Correctly parse tags lacking a description

Fix compilation for mingw32 and cygwin

inet_pton is available only in windows vista or later,
fixed the issue by reimplementing it using WSAStringToAddress

fetch: Write packs to the ODB instead of directly to disk

Bail out of remote loading if the URL would be NULL

Apparently git_remote_list() includes even remotes for which git_remote_load() would fail. Sorry @nulltoken, false alarm.

This reverts commit f358ec14.

CC @nulltoken

Fix state when HEAD is not detached

Let the user grab the packfile as it's being written

This fixes a crash from attempting to invoke git__strdup() against NULL.