copies: move comment about implementation of mergecopies() to end

When you start reading about mergecopies(), you want to know what it
is, not that there are different implementations depending on config,
so this patch moves that comment to the end.

By the way, we don't seem to define what "copytracing" is. I'm just
leaving it that way because I don't know what it is myself. It seems
to be referred to only on mergecopies() (and not in pathcopies(), for
example), so maybe "copytracing" is supposed to be exactly what
mergecopies() does?

Differential Revision: https://phab.mercurial-scm.org/D6235

mercurial/copies.py

@@ -430,8 +430,7 b' def _combinecopies(copyfrom, copyto, fin'
 
 def mergecopies(repo, c1, c2, base):
     """
-    The function calling different copytracing algorithms on the basis of config
-    which find moves and copies between context c1 and c2 that are relevant for
+    Finds moves and copies between context c1 and c2 that are relevant for
     merging. 'base' will be used as the merge base.
 
     Copytracing is used in commands like rebase, merge, unshelve, etc to merge
@@ -472,6 +471,8 b' def mergecopies(repo, c1, c2, base):'
     "dirmove" is a mapping of detected source dir -> destination dir renames.
     This is needed for handling changes to new files previously grafted into
     renamed directories.
+
+    This function calls different copytracing algorithms based on config.
     """
     # avoid silly behavior for update from empty dir
     if not c1 or not c2 or c1 == c2: