Add a new branch to the `testrepo` repository, where the `README` file
has changed to executable.  This branch enables typechange tests between
the new `executable` branch and `master`.

The test `iterator::workdir::filesystem_gunk` is usually not executed,
as it is guarded by the environment variable "GITTEST_INVASIVE_SPEED"
due to its effects on speed. As such, it has become stale and does not
account for new references which have meanwhile been added to the
testrepo, causing it to fail. Fix this by raising the number of expected
references to 15.

Some function bodies of tests which are not applicable to the Win32
platform are completely #ifdef'd out instead of calling `cl_skip()`.
This leaves us with no indication that these tests are not being
executed at all and may thus cause decreased scrutiny when investigating
skipped tests. Improve the situation by calling `cl_skip()` instead of
just doing nothing.

Add a new branch that causes a merge conflict to `testrepo` so
that we are able to test merging in worktrees.

When running as root, skip the unreadable file tests, because, well,
they're probably _not_ unreadable to root unless you've got some
crazy NSA clearance-level honoring operating system shit going on.

Allow callers to specify a start path with a trailing slash to match
a submodule, instead of just a directory.  This is for some legacy
behavior that's sort of dumb, but there it is.

(It's slow!)

Iterator tests were split over repo::iterator and diff::iterator,
with duplication between the two.  Move them to iterator::index,
iterator::tree, and iterator::workdir.