merge: don't call copies.mergecopies unless we need to

This reduces the amount of time we spend calculating when doing a clean
non-merge update. In a large repo, the time dropped from 10.1 seconds
to 3.4.

mercurial/merge.py

@@ -200,6 +200,8 b' def manifestmerge(repo, wctx, p2, pa, br'
         pa = wctx
     elif pa == p2: # backwards
         pa = wctx.p1()
+    elif not branchmerge and not wctx.dirty(missing=True):
+        pass
     elif pa and repo.ui.configbool("merge", "followcopies", True):
         ret = copies.mergecopies(repo, wctx, p2, pa)
         copy, movewithdir, diverge, renamedelete = ret

tests/test-subrepo.t

@@ -210,7 +210,6 b' merge tests'
   subrepo merge f0d2028bf86d+ 1831e14459c4 1f14a2e2d3ec
     subrepo t: other changed, get t:6747d179aa9a688023c4b0cad32e4c92bb7f34ad:hg
   getting subrepo t
-    searching for copies back to rev 1
   resolving manifests
    branchmerge: False, force: False, partial: False
    ancestor: 60ca1237c194, local: 60ca1237c194+, remote: 6747d179aa9a