context: introduce an `isintroducedafter` method and use it in copies

Right now, copy tracing make effort to not traverse the graph too much to save
performance. It uses a "limit" acting as a floor revision past which data are
no longer relevant to the current copy tracing.

However, to enforce this limit, it does a call to `filectx.rev()` and that
call can trigger a graph traversal on its own. That extra graph traversal is
unaware of the current limit and can become very expensive. That cost is
increased by the nature of work done in adjust link rev, we are not only
walking down the graph, we are also checking the affected file for each
revision we walk through. Something significantly more expensive than the walk
itself.

To work around this we need to make the `filectx` operation aware of the
current limit. The first step is to introduce a dedicated method:
`isintroducedafter`. We'll then rework that method logic to stop traversal as
soon as possible.

mercurial/context.py

@@ -760,6 +760,12 b' class basefilectx(object):'
             # result is crash somewhere else at to some point.
         return lkr
 
+    def isintroducedafter(self, changelogrev):
+        """True if a filectx has been introduced after a given floor revision
+        """
+        return (self.linkrev() >= changelogrev
+                or self.introrev() >= changelogrev)
+
     def introrev(self):
         """return the rev of the changeset which introduced this file revision
 

mercurial/copies.py

@@ -139,7 +139,7 b' def _tracefile(fctx, am, limit=-1):'
     for f in fctx.ancestors():
         if am.get(f.path(), None) == f.filenode():
             return f
-        if limit >= 0 and f.linkrev() < limit and f.rev() < limit:
+        if limit >= 0 and not f.isintroducedafter(limit):
             return None
 
 def _dirstatecopies(d, match=None):