Detect tags from commit ids for conflict resolution

[mattfarina]

When resolving conflicts and possibly importing from others lock files detect tags associated with a listed commit. This will aide automating the SemVer handling.

See Masterminds/vcs#19 for implementation details with the VCS. This can be done fairly easily.

[technosophos]

That is a fabulous idea. Knowing that a commit ties to a tag actually makes life much easier for us.

[sdboyer]

big ups here. support for this is also on my TODO list in the solver.

ideally, the lock files generated should include both a tag (when applicable) and a revision. it doesn't actually obviate the need for this check - it mostly allows us to detect if/when upstream does something naughty and moves a tag. so...sorta related to this.

[mattfarina]

It's now displayed. See #459.