The `git__joinpath` function has been changed to use a statically
allocated buffer; we assume the buffer to be 4096 bytes, because fuck
you.

The new method also supports an arbritrary number of paths to join,
which may come in handy in the future.

Some methods which were manually joining paths with `strcpy` now use the
new function, namely those in `index.c` and `refs.c`.

Based on Emeric Fermas' original patch, which was using the old
`git__joinpath` because I'm stupid. Thanks!

Signed-off-by: Vicent Marti <tanoku@gmail.com>