Switched to using git for Windows, rather than msysgit.

Using msysgit with git-annex is no longer supported.

At the same time, I'm updating the rsync.exe in my downloads repository
with the one from msys2.

Note that rsync is currently still being ldded and installed in Git/cmd/
like the other cygwin programs. The ldd fails and this failure is ignored.
It would be better to special case it to go in Git/usr/bin/, so that the
user can't run rsync in a dos prompt window, which doesn't work, as it needs
additional libs. However, as far as git-annex running rsync running ssh,
it works ok in this location.

Removed the ssh.cmd and ssh-keygen.cmd; these are not needed with git for
windows. Keeping them would let ssh be run manually from a dos prompt
window, but that's not really a goal.

debian/changelog

@@ -12,6 +12,8 @@ git-annex (5.20150825) UNRELEASED; urgency=medium
   * When gpg.program is configured, it's used to get the command to run
     for gpg. Useful on systems that have only a gpg2 command or want to
     use it instead of the gpg command.
+  * Windows: Switched to using git for Windows, rather than msysgit.
+    Using msysgit with git-annex is no longer supported.
 
  -- Joey Hess <id@joeyh.name>  Tue, 01 Sep 2015 14:46:18 -0700