version-gen: cleanup

No functional changes.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
version-gen: fix versions

Virtually all packaging guidelines would prefer 1.8.4~rc1, over
1.8.4.rc1 or 1.8.4-rc1, so it makes sense to use that instead.

In particular, the only packaging we provide, git.spec, generates a
wrong version, because git-1.8.4 < git-1.8.4.rc1, changing to ~rc1 fixes
the problem as it's considered newer.

The same happens in dpkg.

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>