revert: handle subrepos missing in the given --rev

The list of subrepos to revert is currently based on the given --rev, so there
is currently no way for this to fail.  Using the --rev context is wrong though,
because if the subrepo doesn't exist in --rev, it is skipped, so it won't be
changed.  This change makes it so that the revert aborts, which is what happens
if a plain file is reverted to -1.  Finding matches based on --rev is also
inconsistent with evaluating files against the working directory (5b85a5bc5bbb).

This change is made now, so as to not cause breakage when the context is
switched in an upcoming patch.

mercurial/cmdutil.py

@@ -3055,7 +3055,11 b' def revert(ui, repo, ctx, parents, *pats'
         if targetsubs:
             # Revert the subrepos on the revert list
             for sub in targetsubs:
-                ctx.sub(sub).revert(ctx.substate[sub], *pats, **opts)
+                try:
+                    ctx.sub(sub).revert(ctx.substate[sub], *pats, **opts)
+                except KeyError:
+                    raise util.Abort("subrepository '%s' does not exist in %s!"
+                                      % (sub, short(ctx.node())))
     finally:
         wlock.release()