py3: replace file() with open() in test-patch-offset.t
file() is not present in Python 3.
Differential Revision: https://phab.mercurial-scm.org/D2119
#require test-repo jshint hg10
$ . "$TESTDIR/helpers-testrepo.sh"
run jshint on all tracked files ending in .js except vendored dependencies
$ cd "`dirname "$TESTDIR"`"
$ testrepohg locate 'set:**.js' \
> 2>/dev/null \
> | xargs jshint