copies: extract function for deciding whether to use changeset-centric algos

We'll eventually have a "experimental.copies.read-from=changeset-only"
option too and I don't want to spread the logic for determining if we
should use changeset-centric of filelog-centric algorithms.

Differential Revision: https://phab.mercurial-scm.org/D6163

mercurial/copies.py

@@ -160,13 +160,18 b' def _computeforwardmissing(a, b, match=N'
     mb = b.manifest()
     return mb.filesnotin(ma, match=match)
 
+def usechangesetcentricalgo(repo):
+    """Checks if we should use changeset-centric copy algorithms"""
+    return (repo.ui.config('experimental', 'copies.read-from') ==
+            'compatibility')
+
 def _committedforwardcopies(a, b, match):
     """Like _forwardcopies(), but b.rev() cannot be None (working copy)"""
     # files might have to be traced back to the fctx parent of the last
     # one-side-only changeset, but not further back than that
     repo = a._repo
 
-    if repo.ui.config('experimental', 'copies.read-from') == 'compatibility':
+    if usechangesetcentricalgo(repo):
         return _changesetforwardcopies(a, b, match)
 
     debug = repo.ui.debugflag and repo.ui.configbool('devel', 'debug.copies')

mercurial/scmutil.py

@@ -1205,7 +1205,7 b' def _markchanges(repo, unknown, deleted,'
             wctx.copy(old, new)
 
 def getrenamedfn(repo, endrev=None):
-    if repo.ui.config('experimental', 'copies.read-from') == 'compatibility':
+    if copiesmod.usechangesetcentricalgo(repo):
         def getrenamed(fn, rev):
             ctx = repo[rev]
             p1copies = ctx.p1copies()