Currently we use the most naïve and inefficient method for figuring out
which objects to send to the remote whereby we end up trying to insert
subdirs which have not changed multiple times.

Instead, make use of the packbuilder's built-in more efficient method
which uses the walk to feed the object list and avoids inserting an
object and its descendants.

Most use-cases for the object packer communicate in terms of commits
which each side has. We already have an object to specify this
relationship between commits, namely git_revwalk.

By knowing which commits we want to pack and which the other side
already has, we can perform similar optimisations to git, by marking
each tree as interesting or uninteresting only once, and not sending
those trees which we know the other side has.

Keep the definitions in the headers, while putting the declarations in
the C files. Putting the function definitions in headers causes
them to be duplicated if you include two headers with them.

Revert "fastfail" from the mingw PR

This reverts commit 8008ab6a.

This reverts commit ba6c53b9.

Fixes

RFC: add mingw to appveyor matrix

refdb: use the same id for old and new when renaming a reference

When we rename a reference, we want the old and new ids to be the same
one (as we did not change it). The normal code path looks up the old id
from the current value of the brtanch, but by the time we look it up, it
does not exist anymore and thus we write a zero id.

Pass the old id explicitly instead.

Filter relative paths

branch: fix generated reflog message upon creation

Test that filter_list_apply_to_file works and can accept repo-relative
paths.

these shouldn't be necessary if _WIN32_WINNT >= _WIN32_WINNT_VISTA

should cut down on compiler warnings with mingw

cache mingw-w64 downloads

quiet curl and 7zip

run appveyor steps in cmd for mingw

use MSYS makefiles generator

add bash script for running mingw on appveyor

add --login and fix run paths

use msys style path to appveyor-mingw.sh

add mingw path to /etc/fstab

Fix crash in git_clone on extremely large repos

Free TLS data on thread exit (win32)

Clear the error message on git_libgit2_shutdown for all versions of
the library (no threads and Win32 threads).  Drop the giterr_clear
in clar, as that shouldn't be necessary.

Set up git_trace in clar test suite.

diff_tform: don't compare empty hashsig_heaps

When comparing seemingly blank files, take whitespace options into
account.

Don't try to compare two empty hashsig_heaps.

http: enforce the credential types