We changed the defaults on strict object creation - it is enabled by
default.  Update the documentation to reflect that.

Fix the existence check for `regcomp_l`.

`xlocale.h` only defines `regcomp_l` if `regex.h` was included as well.

Also change the test cases to actually test `p_regcomp` works with
a multibyte locale.

Improve revision walk preparation logic

When we read from the list which `limit_list()` gives us, we need to check that
the commit is still interesting, as it might have become uninteresting after it
was added to the list.

Make sure we use the `C` locale for `regcomp` on macOS.

`git-rebase--merge` does not ask for time sorting, but uses the default. We now
produce the same default time-ordered output as git, so make us of that since
it's not always the same output as our time sorting.

It changed from implementation-defined to git's default sorting, as there are
systems (e.g. rebase) which depend on this order. Also specify more explicitly
how you can get git's "date-order".

After `limit_list()` we already have the list in time-sorted order, which is
what we want in the "default" case. Enqueueing into the "unsorted" list would
just reverse it, and the topological sort will do its own sorting if it needs
to.