nbgv get-commits doesn't work when version.json specifies x.y.z

[AArnott]

In a repo where the version.json file specifies three integer versions (moving git height to the 4th position, or even elsewhere like a prerelease tag), the nbgv get-commits command returns no commits when the 4 integer version is specified and the wrong commits for the 3 integer version is given.

We should make this smart enough to interpret the version.json file so it knows how to match a version number as well as possible.

While we're at it, it should not puke when a full semver version is specified (including -prerelease tag).