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.

Fix bytes_received in fetch tests - we weren't calling the callback

This fix makes libgit2 capable of parsing annotated tag objects that lack
the optional message/description field.
Previously, libgit2 treated this field as mandatory and raised a tag_error on
such tags. However, the message field is optional.

An example of such a tag is refs/tags/v2.6.16.31-rc1 in Linux:

$ git cat-file tag refs/tags/v2.6.16.31-rc1
object afaa018cefb6af63befef1df7d8febaae904434f
type commit
tag v2.6.16.31-rc1
tagger Adrian Bunk <bunk@stusta.de> 1162716505 +0100
$

Initial implementation of diff rename detection