remote: add some comments to the callback struct

Hopefully clear up what they're for.
reflog: move the reflog implementation into refdb_fs

References and their logs are logically coupled, let's make it so in
the code by moving the fs-based reflog implementation to live next to
the fs-based refs one.

As part of the change, make the function take names rather than
references, as only the names are relevant when looking up and
handling reflogs.
reflog: add a convenience append function

Provide a function that reads a reflog, performs an append and writes back to the
backend in one call.
reflog: bring _append and _drop back to the frontend

These functions act purely on the reflog data structure.
clone: put the callbacks struct directly in the clone options

There's no need for this to be a pointer to somewhere else.
Merge pull request #1855 from libgit2/cmn/clone-into

An alternative for 'clone' with options
Support cancellation in push operation

This commit adds cancellation for the push operation. This work consists of:

1) Support cancellation during push operation
    - During object counting phase
    - During network transfer phase
        - Propagate GIT_EUSER error code out to caller
2) Improve cancellation support during fetch
    - Handle cancellation request during network transfer phase
    - Clear error string when cancelled during indexing
3) Fix error handling in git_smart__download_pack

Cancellation during push is still only handled in the pack building and
network transfer stages of push (and not during packbuilding).
Fix broken online push tests
HTTP: handle "relative" redirects
Merge pull request #1889 from libgit2/relative-redirect

HTTP: handle "relative" redirects
commit: Introduce git_commit_message_raw()
Merge pull request #1887 from libgit2/ntk/topic/git_message_raw

commit: Introduce git_commit_message_raw()
Merge branch 'development' into blame

Conflicts:
	include/git2.h
Style tweaks and changes for code review feedback
Merge pull request #1888 from jamill/network_cancellation

network cancellation improvements
Add check if we need to precompose unicode on Mac

This adds initialization of core.precomposeunicode to repo init
on Mac.  This is necessary because when a Mac accesses a repo on
a VFAT or SAMBA file system, it will return directory entries in
decomposed unicode even if the filesystem entry is precomposed.

This also removes caching of a number of repo properties from the
repo init pipeline because these are properties of the specific
filesystem on which the repo is created, not of the system as a
whole.
Put hooks in place for precompose in dirload fn

This doesn't actual do string precompose but it puts the hooks in
place into the iterators and the git_path_dirload function so that
the actual precompose work is ready to go.
Initial iconv hookup for precomposed unicode

This hooks up git_path_direach and git_path_dirload so that they
will take a flag indicating if directory entry names should be
tested and converted from decomposed unicode to precomposed form.
This code will only come into play on the Apple platform and even
then, only when certain types of filesystems are used.

This involved adding a flag to these functions which involved
changing a lot of places in the code.

This was an opportunity to do a bit of code cleanup here and there,
for example, getting rid of the git_futils_cleanupdir_r function in
favor of a simple flag to git_futils_rmdir_r to not remove the top
level entry.  That ended up adding depth tracking during rmdir_r
which led to a safety check for infinite directory recursion.  Yay.

This hasn't actually been tested on the Mac filesystems where the
issue occurs.  I still need to get test environment for that.
Add to Git authors who have agreed to relicense

In email, Torsten Bögershausen agreed that we could use his code
from core Git in libgit2 under the modified license.  Also, since
his work is the basis for much of the precompose unicode support,
I have added him to the AUTHORS file as well.
Simplify git_path_is_empty_dir implementation

This simplifies git_path_is_empty_dir on both Windows (getting rid
of git_buf allocation inside the function) and other platforms (by
just using git_path_direach), and adds tests for the function, and
uses the function to simplify some existing tests.