Common

I realize this looks like silly yak shaving, or worse, but if one were to be super crazy and decide that they spent a lot of time working with libgit2 and had decided that things like `git_futils_*` were really handy xplat APIs, then they might decide  they wanted to make use of them in other projects.

Rationale:
I've been poking around using some of the functions in `util.c` in another project - something like [jagged](https://github.com/ethomson/jagged) - and it would be great to just include that file since I'm already pulling the libgit2 source in with a submodule.