Merge branch 'mg/work-tree-tests' into maint

Traditionally, the tests that try commands that work on the
contents in the working tree were named with "worktree" in their
filenames, but with the recent addition of "git worktree"
subcommand, whose tests are also named similarly, it has become
harder to tell them apart.  The traditional tests have been renamed
to use "work-tree" instead in an attempt to differentiate them.

* mg/work-tree-tests:
  tests: rename work-tree tests to *work-tree*