mergecopies: rename 'ca' to 'base'

mergecopies: 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. Similar rename
was also applied to 'checkcopies' in a prior changeset.

--- a/mercurial/copies.py	Thu Oct 13 01:26:33 2016 +0200
+++ b/mercurial/copies.py	Thu Oct 13 01:30:14 2016 +0200
@@ -285,10 +285,10 @@
         return fctx
     return util.lrucachefunc(makectx)
 
-def mergecopies(repo, c1, c2, ca):
+def mergecopies(repo, c1, c2, base):
     """
     Find moves and copies between context c1 and c2 that are relevant
-    for merging.
+    for merging. 'base' will be used as the merge base.
 
     Returns four dicts: "copy", "movewithdir", "diverge", and
     "renamedelete".
@@ -329,7 +329,7 @@
 
     m1 = c1.manifest()
     m2 = c2.manifest()
-    ma = ca.manifest()
+    mb = base.manifest()
 
     # gather data from _checkcopies:
     # - diverge = record all diverges in this dict
@@ -346,17 +346,17 @@
             }
 
     # find interesting file sets from manifests
-    addedinm1 = m1.filesnotin(ma)
-    addedinm2 = m2.filesnotin(ma)
+    addedinm1 = m1.filesnotin(mb)
+    addedinm2 = m2.filesnotin(mb)
     u1r, u2r = _computenonoverlap(repo, c1, c2, addedinm1, addedinm2)
     u1u, u2u = u1r, u2r
     bothnew = sorted(addedinm1 & addedinm2)
 
     for f in u1u:
-        _checkcopies(c1, f, m1, m2, ca, limit, data1)
+        _checkcopies(c1, f, m1, m2, base, limit, data1)
 
     for f in u2u:
-        _checkcopies(c2, f, m2, m1, ca, limit, data2)
+        _checkcopies(c2, f, m2, m1, base, limit, data2)
 
     copy = dict(data1['copy'].items() + data2['copy'].items())
     fullcopy = dict(data1['fullcopy'].items() + data2['fullcopy'].items())
@@ -384,8 +384,8 @@
                 'diverge': bothdiverge,
                }
     for f in bothnew:
-        _checkcopies(c1, f, m1, m2, ca, limit, bothdata)
-        _checkcopies(c2, f, m2, m1, ca, limit, bothdata)
+        _checkcopies(c1, f, m1, m2, base, limit, bothdata)
+        _checkcopies(c2, f, m2, m1, base, limit, bothdata)
     for of, fl in bothdiverge.items():
         if len(fl) == 2 and fl[0] == fl[1]:
             copy[fl[0]] = of # not actually divergent, just matching renames