update on libgit2

[Dirk Hohndel]

I have been working with one of the developers on the libgit2 team and am
happy to announce that with his code libgit2 successfully uses an http
proxy and I can connect to the Subsurface cloud storage infrastructure
through a proxy.

Since this hasn't landed it libgit2 master, yet, I'm not quite ready to
push the code changes on the Subsurface side, but this is solving a huge
concern of mine regarding the viability of cloud storage in Subsurface.

It will mean that Subsurface 4.5 needs the very bleeding edge of libgit2
which of course has some issues of its own. But you win some, you lose
some :-)

/D