This bumps subsurface to 4.9.3.
As the subsurface-specific libdivecomputer source is shipped as a git
submodule these days, we can avoid having to fetch a separate tarball
that needs to be kept in sync (and is only uploaded for each release).
This also applies a patch on top of 4.9.3 to properly handle libgit2
versions 1.0 and later - conditional code for older libgit versions has
been removed upstream, but that's a nontrivial backport, which is why
this patch was added.
