`git__timer` is now `git_time_monotonic`, and returns milliseconds
since an arbitrary epoch.

Using a floating point to store the number of seconds elapsed was
clever, as it better supports the wide range of precision from the
different monotonic clocks of different systems. But we're a version
control system, not a real-time clock.

Milliseconds is a good enough precision for our work _and_ it's the
units that system calls like `poll` take and that our users interact
with.

Make `git_time_monotonic` return the monotonically increasing number
of milliseconds "ticked" since some arbitrary epoch.

Including the `include/git2` build path is a seemingly unnecessary
oversight to include the generated `experimental.h` file.

SHA256: more SHA256 support

Fix build failure with -DEMBED_SSH_PATH

This is much of the plumbing for the object database to support SHA256,
and for objects to be able to parse SHA256 versions of themselves.

The experimental function signature is only available when
`GIT_EXPERIMENTAL_SHA256` is enabled.

libgit2 can be built with optional, experimental sha256 support. This
allows consumers to begin testing and providing feedback for our sha256
support while we continue to develop it, and allows us to make API
breaking changes while we iterate on a final sha256 implementation.

The results will be `git2-experimental.dll` and installed as
`git2-experimental.h` to avoid confusion with a production libgit2.