util: enable getpid to be replaced This will enable tests to write stable process ids.

blackbox: refactor use of vfs as _bbvfs

blackbox: flush output file descriptor Without this, when there are multiple ui views, each blackbox will have its own file handle, and the logging will be in a really bad order. Also, because of the way blackbox works, it never closes its file handles, which means the last output before exit is often lost.

tests: change blackbox test to work cross platform While it is not easy to make a file 000 on Windows, you can emulate most of the behaviors by replacing the file with a directory. Also corrects test description to properly indicate that failing to read from the log is fatal.

merge: document checkignored and checkunknown configs again These options were undocumented for 3.7 because of an issue found during the freeze (see rev 7cb7264cfd52). This issue has now been fixed, so we can document these options again.

rebase: respect checkunknown and checkignored in more cases checkunknown and checkignored are currently respected for updates and regular merges, but not for certain kinds of rebases. To be precise, they aren't respected for rebases when: (1) we're rebasing while currently on the destination commit, and (2) an untracked or ignored file F is currently in the working copy, and (3) the same file F is in a source commit, and (4) F has different contents in the source commit. This happens because rebases set force to True when calling merge.update. Setting force to True makes a lot of sense in general, but it turns out the force option is overloaded: there's a deprecated '--force' option in merge that allows you to merge in outstanding changes, including changes in untracked files. We use the 'mergeforce' parameter to tell those two cases apart. I think the behavior during rebases when checkunknown is 'abort' (the default) is wrong -- we should abort on or overwrite differing untracked files, not try to merge them in. However that currently breaks rebases by aborting in the middle -- we need better handling for that case before we can change the default.