largefiles: force an on-disk merge Largefiles isn't a good candidate for in-memory merge (it uses a custom dirstate, matcher, and the files might not fit in memory) so have it always run an old-style merge. Differential Revision: https://phab.mercurial-scm.org/D683

merge: allow a custom working context to be passed to update This will allow anyone to enable the first in-menmory merge milestone by wrapping merge.update in an extension and creating an overlayworkingctx. Differential Revision: https://phab.mercurial-scm.org/D682

patchbomb: add test that shows --to and --cc override matching config item As far as I know this has always been true and is intentional (it's in line with many other behaviors), but it wasn't tested.

blackbox: move _openlogfile to a separate method This removes several `stat` syscalls for accessing `_bbvfs` and makes `ui` object cleaner. Differential Revision: https://phab.mercurial-scm.org/D769

blackbox: do not prevent 'chg init' from working Previously, blackbox always appends to blackbox.log and creates the directory for that file on demand. That could be an issue if: 1. chg starts from `$REPO` directory, so `ui._bbrepo` is set. 2. `rm -rf $REPO`. 3. `chg init $REPO`, blackbox writes something and `init` will fail because `$REPO` directory is non-empty. This patch fixes that by verifying whether vfs exists before re-using it. Differential Revision: https://phab.mercurial-scm.org/D768

tests: add a test for blackbox with nested alias configurations I've observed some weirdness around this, and needed to rule some things out. There aren't any bugs in core around this, but it was nice to have confirmation.

tests: clean up blackbox test around aliases a little bit