megarepo: handle "pull -r <xrepo commit>"

Summary:
Support the `rewritepullrev` option for the megarepo cross-repo commit resolution. This incurs an extra remote call even if the "-r" rev belongs to the default remote. We have no way to know a priori if a locally-unknown commit is available in our default remote or not.

An alternative would be to only trigger autopull after the regular pull has failed (that would avoid the extra network request if the regular pull worked). That would be more complicated though, so let's start with this.

Reviewed By: zzl0

Differential Revision: D55045406

fbshipit-source-id: db842a5810eb912c173226d181520eaee0249f96

eden/scm/sapling/ext/megarepo.py

@@ -67,8 +67,8 @@ def cachedname(repo, commitid):
     )
 
 
-@autopullpredicate("megarepo", priority=100)
-def _xrepopull(repo, name) -> deferredpullattempt:
+@autopullpredicate("megarepo", priority=100, rewritepullrev=True)
+def _xrepopull(repo, name, rewritepullrev=False) -> Optional[pullattempt]:
     """Autopull a commit from another repo.
 
     First the xrepo commit is translated to the coresponding commit of
@@ -86,7 +86,13 @@ def generateattempt() -> Optional[pullattempt]:
             return None
         return autopull.pullattempt(headnodes=[localnode])
 
-    return deferredpullattempt(generate=generateattempt)
+    if rewritepullrev:
+        if repo.ui.configbool("megarepo", "rewrite-pull-rev", True):
+            return generateattempt()
+        else:
+            return None
+    else:
+        return deferredpullattempt(generate=generateattempt)
 
 
 _commithashre = re.compile(r"\A[0-9a-f]{6,40}\Z")