diff: pass a diff hunks filter function from changeset_printer to patch.diff() We add a 'hunksfilterfn' keyword argument in all functions of the call stack from changeset_printer.show() to patch.diff(). This is a callable that will be used to filter out hunks by line range and will be used in the "-L/--line-range" option of "hg log" command introduced in the following changesets.

diff: also yield file context objects in patch.trydiff() (API) And retrieve them in patch.diffhunks(). We'll use these in forthcoming changesets to filter diff hunks by line range.

revset: extract a parsefollowlinespattern helper function We'll need the same logic in forthcoming changeset to handle --line-range option in 'hg log' command. The function lives in scmutil.py (rather than util.py) as it uses match and pathutil modules.