indexer: check the packfile trailer

The packfile trailer gets sent over and we should check whether it's
correct as part of our sanity checks of the packfile.

Configurable template dir for Win32

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.

This adds more tests of filters, including the ident filter when
mixed with custom filters.  I was able to combine with the reverse
filter and demonstrate that the order of filter application with
the default priority constants matches the order of core Git.

Also, this fixes two issues in the ident filter: preventing ident
expansion on binary files and avoiding a NULL dereference when
dollar sign characters are found without Id.

Fixed the filter order to match core Git, too.

This test demonstrates an interesting behavior of core Git (which
is totally reasonable and which libgit2 matches, although mostly
by coincidence).  If you use the ident filter and commit a file
with a garbage ident in it, like '$Id: this is just garbage$' and
then immediately do a 'git checkout-index' with the new file, Git
will not consider the file out of date and will not overwrite the
file with an updated $Id$.  Libgit2 has the same behavior.  If you
remove the file and then do a checkout-index, it will be replaced
with a filtered version that has injected the OID correctly.