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).
Our full contribution guidelines are in our wiki, please see:
https://www.mercurial-scm.org/wiki/ContributingChanges
If you just want a checklist to follow, you can go straight to
https://www.mercurial-scm.org/wiki/ContributingChanges#Submission_checklist
If you can't run the entire testsuite for some reason (it can be
difficult on Windows), please at least run `contrib/check-code.py` on
any files you've modified and run `python contrib/check-commit` on any
commits you've made (for example, `python contrib/check-commit
273ce12ad8f1` will report some style violations on a very old commit).