Gitaly is in the process of migrating UserSquash to use detailed errors
like we already do for UserMergeBranch. This change will break the
current calling convention, where Gitaly will return some errors by
returning successfully, but with an `git_error` field set in the
response. Instead, Gitaly will return a proper error for all conditions
but give clients the ability to distinguish error cases via error
details.

Prepare the OperationService for this change by handling those new error
details. Theoretically, this allows us to implement more fine-grained
handling of these errors now: `ResolveRevisionErrors` contain the
revision that Gitaly tried to resolve but couldn't, and
`MergeConflictErrors` contain information about the files that have been
conflicting. To ease the transition though we're treating those errors
exactly the same as we did previously, where we raised a `GitError`. We
can still iterate on this at a later point.

The end result should be that there is no real user-visible change in
how we handle UserSquash.