copies: document hack for adding '' to set of dirs

The root directory is not normally added to 'dirs' instances (although
I think it should be). In copies.mergecopies, we call dirname() to get
the directory of a path and then check for containment in the 'dirs'
instances ('d1' and 'd2'). In order to easily handle files in the root
directory, '/' is added to d1/d2. This results in the empty string
being added to the sets, since what comes before the slash in '/' is
an empty string. This seems less than obvious, so let's document it.

mercurial/copies.py

@@ -370,6 +370,7 b' def mergecopies(repo, c1, c2, ca):'
 
     # generate a directory move map
     d1, d2 = c1.dirs(), c2.dirs()
+    # Hack for adding '', which is not otherwise added, to d1 and d2
     d1.addpath('/')
     d2.addpath('/')
     invalid = set()