checkcopies: rename 'ca' to 'base'

checkcopies: rename 'ca' to 'base'

This variable was named after the common ancestor. It is actually the merge
base that might differ from the common ancestor in the graft case. We rename the
variable before a larger refactoring to clarify the situation.

--- a/mercurial/copies.py	Wed Aug 24 05:09:46 2016 +0200
+++ b/mercurial/copies.py	Sat Oct 08 18:38:42 2016 +0200
@@ -454,7 +454,7 @@
 
     return copy, movewithdir, diverge, renamedelete
 
-def _checkcopies(ctx, f, m1, m2, ca, limit, diverge, copy, fullcopy):
+def _checkcopies(ctx, f, m1, m2, base, limit, diverge, copy, fullcopy):
     """
     check possible copies of f from m1 to m2
 
@@ -462,7 +462,7 @@
     f = the filename to check
     m1 = the source manifest
     m2 = the destination manifest
-    ca = the changectx of the common ancestor
+    base = the changectx used as a merge base
     limit = the rev number to not search beyond
     diverge = record all diverges in this dict
     copy = record all non-divergent copies in this dict
@@ -474,7 +474,7 @@
     once it "goes behind a certain revision".
     """
 
-    ma = ca.manifest()
+    mb = base.manifest()
     getfctx = _makegetfctx(ctx)
 
     def _related(f1, f2, limit):
@@ -523,15 +523,15 @@
         fullcopy[f] = of # remember for dir rename detection
         if of not in m2:
             continue # no match, keep looking
-        if m2[of] == ma.get(of):
+        if m2[of] == mb.get(of):
             return # no merge needed, quit early
         c2 = getfctx(of, m2[of])
-        cr = _related(oc, c2, ca.rev())
+        cr = _related(oc, c2, base.rev())
         if cr and (of == f or of == c2.path()): # non-divergent
             copy[f] = of
             return
 
-    if of in ma:
+    if of in mb:
         diverge.setdefault(of, []).append(f)
 
 def duplicatecopies(repo, rev, fromrev, skiprev=None):