Martin von Zweigbergk <martinvonz@google.com> [Fri, 22 May 2015 14:02:04 -0700] rev 25288
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.