A new FileType, GitSubmodule is added which is ignored. Files or
directories having this type are not added to the work queue and
are ignored in snapshot. Submodules are not created by jujutsu
when resetting or checking out a tree, they should be currently
managed using git.
Let's acknowledge everyone's contributions by replacing "Google LLC"
in the copyright header by "The Jujutsu Authors". If I understand
correctly, it won't have any legal effect, but maybe it still helps
reduce concerns from contributors (though I haven't heard any
concerns).
Google employees can read about Google's policy at
go/releasing/contributions#copyright.
There are no "non-normal" files, so "normal" is not needed. We have
symlinks and conflicts, but they are not files, so I think just "file"
is unambiguous.
I left `testutils::write_normal_file()` because there it's used to
mean "not executable file" (there's also a `write_executable_file()`).
I left `working_copy::FileType::Normal` since renaming `Normal` there
to `File` would also suggest we should rename `FileType`, and I don't
know what would be a better name for that type.
I was reading a draft of "Git Rev News: Edition 91" [1] where Peff
mentions some unfinished patches to allow negative timestamps in
Git. So I figured I should add support for that before I forget. I
haven't checked if libgit2 supports it, so it might be that our Git
backend still doesn't support it after this patch.
[1] https://github.com/git/git.github.io/blob/master/rev_news/drafts/edition-91.md
`wc_commit` seems clearer than `checkout` and not too much longer. I
considered `working_copy` but it was less clear (could be the path to
the working copy, or an instance of `WorkingCopy`). I also considered
`working_copy_commit`, but that seems a bit too long.
