tests/test-gendoc-sv.t

revlog: add a _datareadfp context manager for data access needs

The helper handles:
1) is there a file handle already open that we shall just reuse,
2) is the revlog inlined or not.

Using a context manager for all read access will help setting up file pointer
caching in later changesets.

#require docutils gettext

  $ $TESTDIR/check-gendoc sv
  checking for parse errors