This contains a few bug fixes and some header and API cleanups.

The main API change is that filters should now use GIT_PASSTHROUGH
to indicate that they wish to skip processing a file instead of
GIT_ENOTFOUND.

The bug fixes include a possible out-of-range buffer access in
the ident filter, a filter ordering problem I introduced into the
custom filter tests on Windows, and a filter buf NUL termination
issue that was coming up on Linux.