patch: emit deflated binary patches (optionally)

This adds a basic test of doing simultaneous diffs on multiple
threads and adds basic locking for the attr file cache because
that was the immediate problem that arose from these tests.

This is just laying some groundwork for internal index changes
that I'm working on.

The internal buffer in the `git_path_iconv_t` structure was not
being reset before the calls to `iconv` were made to convert data,
so if there were multiple decomposed Unicode paths in a single
directory, paths after the first one were being appended to the
first instead of treated as independent data.

There are a few places where we need to join three strings to
assemble a path.  This adds a simple join3 function to avoid the
comparatively expensive join_n (which calls strlen on each string
twice).