Basic bit vector

This is a simple bit vector object that is not resizable after
the initial allocation but can be of arbitrary size.  It will
keep the bti vector entirely on the stack for vectors 64 bits
or less, and will allocate the vector on the heap for larger
sizes.  The API is uniform regardless of storage location.

This is very basic right now and all the APIs are inline functions,
but it is useful for storing an array of boolean values.
Add git_pathspec_match_diff API

This adds an additional pathspec API that will match a pathspec
against a diff object.  This is convenient if you want to handle
renames (so you need the whole diff and can't use the pathspec
constraint built into the diff API) but still want to tell if the
diff had any files that matched the pathspec.

When the pathspec is matched against a diff, instead of keeping
a list of filenames that matched, instead the API keeps the list
of git_diff_deltas that matched and they can be retrieved via a
new API git_pathspec_match_list_diff_entry.

There are a couple of other minor API extensions here that were
mostly for the sake of convenience and to reduce dependencies
on knowing the internal data structure between files inside the
library.