py3: replace file() with open() in test-convert-hg-source.t
file() is not present in Python 3.
It also makes sure we read and write in bytes mode on Python 3.
Differential Revision: https://phab.mercurial-scm.org/D2132
$ hg init
$ touch unknown
$ touch a
$ hg add a
$ hg ci -m "1"
$ touch b
$ hg add b
$ hg ci -m "2"
Should show unknown
$ hg status
? unknown
$ hg revert -r 0 --all
removing b
Should show unknown and b removed
$ hg status
R b
? unknown
Should show a and unknown
$ ls
a
unknown