NuPkg versions should include commit ID in version for more scenarios

[AArnott]

The current behavior is:

	Stable	Prerelease
PublicRelease=true	1.2.3	1.2.3-pre
PublicRelease!=true	1.2.3	1.2.3-pre-gCommit

The commit ID is not exposed frequently enough to be reliably present and useful.

The current proposal is that we leverage NuGet v3's support for +buildMetadata on the package version to enhance it to this:

	Stable	Prerelease
PublicRelease=true	1.2.3	1.2.3-pre
PublicRelease!=true	1.2.3+gCommit	1.2.3-pre-gCommit

Consider being even more aggressive:

	Stable	Prerelease
PublicRelease=true	1.2.3+gCommit	1.2.3-pre+gCommit
PublicRelease!=true	1.2.3+gCommit	1.2.3-pre-gCommit

This may need to start under an optional property that folks can control. We at least need to weigh the impact on the user experience and whether these packages can be consumed by NuGet v2.x clients.

[AArnott]

Please feel free to comment here or discuss on gitter.im.

[AArnott]

Using +buildMetadata is at least somewhat blocked by NuProj's and NuGet's lack of support for it.

[AArnott]

We really need commit IDs in more places. So the new proposal is:

	Stable	Prerelease
PublicRelease=true	1.2.3	1.2.3-pre
PublicRelease!=true	1.2.3-gCommit	1.2.3-pre-gCommit