merge: check current wc branch for 'nothing to merge', not its p1

The working directory will usually be clean or very clean, and wc will usually
have the same branch as its parent. This change will thus usually not make any
difference and is done as a separate change to show that. It will be used in a
later change.

mercurial/merge.py

@@ -1544,7 +1544,7 b' def update(repo, node, branchmerge, forc'
                 raise error.Abort(_("merging with a working directory ancestor"
                                    " has no effect"))
             elif pas == [p1]:
-                if not mergeancestor and p1.branch() == p2.branch():
+                if not mergeancestor and wc.branch() == p2.branch():
                     raise error.Abort(_("nothing to merge"),
                                      hint=_("use 'hg update' "
                                             "or check 'hg heads'"))