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
$ hg init
$ echo 0 > a
$ echo 0 > b
$ hg ci -A -m m
adding a
adding b
$ hg rm a
$ hg cat a
0
$ hg cat --decode a # more tests in test-encode
0
$ echo 1 > b
$ hg ci -m m
$ echo 2 > b
$ hg cat -r 0 a
0
$ hg cat -r 0 b
0
$ hg cat -r 1 a
a: no such file in rev 7040230c159c
[1]
$ hg cat -r 1 b
1