util: make str(url) return file:/// for abs paths again
str(url) was recently changed to return only file:/. However, the
canonical way to represent absolute local paths is file:/// [1], which
is also expected by at least hgsubversion.
Relative paths are returned as file:the/relative/path.
[1] http://en.wikipedia.org/wiki/File_URI_scheme
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda