This change:
* Stops running tests under Go 1.1 and Go 1.2, since Travis no longer
supports them.
* Stops running tests under Go tip. Go 1.8+ requires runtime.KeepAlive()
to keep some of the references alive across CGo invocations, but that
was introduced in Go 1.7, and that would break older Go versions.
* Fixes a formatting bug that was breaking under newer Go releases.
* Uprevs libgit (test) to v0.24.6.
We have our own libgit2 in a submodule, so this is unnecessary. While in
the area, update which version of libgit2 would be downloaded and remove
unnecessary CMake flags.
The cgo directives let us do a lot more than I previously thought, so we
can use this to make the building process of git2go go through the go
tool directly rather than via the script.
libgit2 still needs to be built manually, so we do still require make,
but only for building libgit2. Once that's built, any modifications to
git2go's own code can be built with
go build