context: add a `blockancestors(fctx, fromline, toline)` function This yields ancestors of `fctx` by only keeping changesets touching the file within specified linerange = (fromline, toline). Matching revisions are found by inspecting the result of `mdiff.allblocks()`, filtered by `mdiff.blocksinrange()`, to find out if there are blocks of type "!" within specified line range. If, at some iteration, an ancestor with an empty line range is encountered, the algorithm stops as it means that the considered block of lines actually has been introduced in the revision of this iteration. Otherwise, we finally yield the initial revision of the file as the block originates from it. When a merge changeset is encountered during ancestors lookup, we consider there's a diff in the current line range as long as there is a diff between the merge changeset and at least one of its parents (in the current line range).