Support auto-resolve of shelved changes

[ca-johnson]

When someone kicks a build with a shelved changelist, there might be merge conflicts when unshelving it. For now, we can live with it (resolve locally, re-shelve and try again) but it would be nice to automatically resolve them with -am

Resolve: None: The current workspace files are used for the build.

Resolve: Safe (-as): Accepts the file in the depot if it has the only changes. Accepts the file in the workspace if it has the only changes. Doesn’t resolve if both the depot and workspace files have changed.

Resolve: Merge (-am): Accepts the file in the depot if it has the only changes. Accepts the workspace file if it has the only changes. Merges changes if both the depot and workspace files have changed and there are no conflicts.

Resolve: Force Merge (-af): Accepts the file in the depot if it has the only changes. Accepts the workspace file if it has the only changes. Creates a merged file if both the depot and workspace files have changed, even if there are conflicts. Where there are conflicts, both versions are included with text notations indicating the conflicts.

Resolve: Yours (-ay): -- keep your edits: Uses the file that is in the workspace and ignores the version of the file that is in the depot.

Resolve: Theirs (-at) -- keep shelf content: Replaces the copy of the file in the workspace with the revision that is in the depot, discards any changes in the workspace file.

[improbable-mattchurch]

Providing a shelf that isn't resolved against latest may fail to compile instead of complaining (which I believe is the default behaviour when doing a perforce unshelve). This has caused a few instances of confusion where it wasn't clear why a build was failing to compile.