revset: add a utility for obtaining the source of a given rev

graft, transplant and rebase all embed a different type of source marker in
extra, and each with a different name.  The current implementation of each is
such that there will never be more than one of these markers on a node.

Note that the rebase marker can only be resolved if the source is
still present, which excludes the typical rebase usage (without
--keep) from consideration (unless the resulting bundle in
strip-backup is overlayed). There probably isn't any reason to use
rebase --keep as a substitute for transplant or graft at this point,
but maybe there was at one point and there are even a few rebases in
the hg repo, so it may be of historical interest.

mercurial/revset.py

@@ -186,6 +186,16 b' def getset(repo, subset, x):'
         raise error.ParseError(_("missing argument"))
     return methods[x[0]](repo, subset, *x[1:])
 
+def _getrevsource(repo, r):
+    extra = repo[r].extra()
+    for label in ('source', 'transplant_source', 'rebase_source'):
+        if label in extra:
+            try:
+                return repo[extra[label]].rev()
+            except error.RepoLookupError:
+                pass
+    return None
+
 # operator methods
 
 def stringset(repo, subset, x):