assistant: Avoid startup hang on active *.lock file

assistant: Avoid startup hang on active *.lock file

Avoid hanging at startup when a process has a *.lock file open in the .git
directory.

The goal is to repair stale locks, not wait for all active locks to be
closed. This was causing problems for a non-git process that has its own
lock file in a subdir of .git/.

If .git/index_lock is a non-stale lock, this does let the assistant start
up regardless. Commits by the assistant will then fail, until the process
locking the index finishes. This is not a problem, because the same
behavior could already happen if the assistant is started and then another
process locks the index.

Sponsored-by: the NIH-funded NICEMAN (ReproNim TR&D3) project