refspec: report errors when parsing an invalid refspec

"In-memory" repos are bare by default

Revert "fastfail" from the mingw PR

If a refspec could not be parsed, the git_refspec__parse function would
return an error value but would not provide additional error information
for the callers. This commit amends that function to set a more useful
error message.

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.