[PATCH 5 of 8 V3] context: introduce a `_findchangerev` method to `filectx`

[Boris Feld]

# HG changeset patch
# User Boris Feld <boris.feld at octobus.net>
# Date 1536255775 14400
#      Thu Sep 06 13:42:55 2018 -0400
# Node ID 964fbe39ab182eb0d65830dc87ef39d0382a41fa
# Parent  759af590645e2f0cc6fbed2b7b97ed718e277dc5
# EXP-Topic copy-perf
# Available At https://bitbucket.org/octobus/mercurial-devel/
#              hg pull https://bitbucket.org/octobus/mercurial-devel/ -r 964fbe39ab18
context: introduce a `_findchangerev` method to `filectx`

In the same spirit as `_introrev` we want to introduce a way to limit graph
walking when resolving `filectx.rev()`. We introduce a new internal function for
this purpose.

diff --git a/mercurial/context.py b/mercurial/context.py
--- a/mercurial/context.py
+++ b/mercurial/context.py
@@ -567,16 +567,21 @@ class basefilectx(object):
 
     @propertycache
     def _changeid(self):
+        return self._findchangerev()
+
+    def _findchangerev(self):
         if r'_changeid' in self.__dict__:
-            return self._changeid
+            changeid = self._changeid
         elif r'_changectx' in self.__dict__:
-            return self._changectx.rev()
+            changeid = self._changectx.rev()
         elif r'_descendantrev' in self.__dict__:
             # this file context was created from a revision with a known
             # descendant, we can (lazily) correct for linkrev aliases
-            return self._adjustlinkrev(self._descendantrev)
+            changeid = self._adjustlinkrev(self._descendantrev)
         else:
-            return self._filelog.linkrev(self._filerev)
+            changeid = self._filelog.linkrev(self._filerev)
+        self._changeid = changeid
+        return changeid
 
     @propertycache
     def _filenode(self):
@@ -810,7 +815,7 @@ class basefilectx(object):
             else:
                 return self._adjustlinkrev(rev, inclusive=True)
         else:
-            return self.rev()
+            return self._findchangerev()
 
     def introfilectx(self):
         """Return filectx having identical contents, but pointing to the