ui: add special-purpose atexit functionality
In spite of its longstanding use, Python's built-in atexit code is
not suitable for Mercurial's purposes, for several reasons:
* Handlers run after application code has finished.
* Because of this, the code that runs handlers swallows exceptions
(since there's no possible stacktrace to associate errors with).
If we're lucky, we'll get something spat out to stderr (if stderr
still works), which of course isn't any use in a big deployment
where it's important that exceptions get logged and aggregated.
* Mercurial's current atexit handlers make unfortunate assumptions
about process state (specifically stdio) that, coupled with the
above problems, make it impossible to deal with certain categories
of error (try "hg status > /dev/full" on a Linux box).
* In Python 3, the atexit implementation is completely hidden, so
we can't hijack the platform's atexit code to run handlers at a
time of our choosing.
As a result, here's a perfectly cromulent atexit-like implementation
over which we have control. This lets us decide exactly when the
handlers run (after each request has completed), and control what
the process state is when that occurs (and afterwards).
context: follow all branches in blockdescendants()
In the initial implementation of blockdescendants (and thus followlines(...,
descend=True) revset), only the first branch encountered in descending
direction was followed.
Update the algorithm so that all children of a revision ('x' in code) are
considered. Accordingly, we need to prevent a child revision to be yielded
multiple times when it gets visited through different path, so we skip 'i'
when this occurs. Finally, since we now consider all parents of a possible
child touching a given line range, we take care of yielding the child if it
has a diff in specified line range with at least one of its parent (same logic
as blockancestors()).
pager: set some environment variables if they're not set
Git did this already [1] [2]. We want this behavior too [3].
This provides a better default user experience (like, supporting colors) if
users have things like "PAGER=less" set, which is not uncommon.
The environment variables are provided by a method so extensions can
override them on demand.
[1]: https://github.com/git/git/blob/
6a5ff7acb5965718cc7016c0ab6c601454fd7cde/pager.c#L87
[2]: https://github.com/git/git/blob/
6a5ff7acb5965718cc7016c0ab6c601454fd7cde/Makefile#L1545
[3]: https://www.mercurial-scm.org/pipermail/mercurial-devel/2017-March/094780.html