Support updating a subproject when it isn't in the root of the git repo

[yajo]

Discussed in #449

^{Originally posted by flippedAben September 22, 2021}
I have a Git repo at ~/repo. I have another Git repo at ~/my_copier_template. I tell copier to use ~/my_copier_template to generate a subproject at ~/repo/path/to/S (call it S for short). I track my new subproject in ~/repo by adding and committing the changes.

When I try to update S, it fails at:

copier/copier/main.py

Lines 607 to 610 in b8ff4d8

	if self.subproject.vcs != "git":
	raise UserMessageError(
	"Updating is only supported in git-tracked subprojects."
	)

This is because copier assumes that a subproject is tracked by Git if and only if it is in the root of a git repo, as stated here:

copier/copier/subproject.py

Lines 77 to 80 in b8ff4d8

	def vcs(self) -> Optional[VCSTypes]:
	"""VCS type of the subproject."""
	if is_git_repo_root(self.local_abspath):
	return "git"

But, S is not at the root of ~/repo. I would like to change copier to support this scenario, but wanted to get some thoughts before proceeding.

Part of the change will involve changing how we update:

copier/copier/main.py

Lines 670 to 673 in b8ff4d8

	git("fetch", "--depth=1", "real_dst", "HEAD")
	diff_cmd = git["diff-tree", "--unified=1", "HEAD...FETCH_HEAD"]
	try:
	diff = diff_cmd("--inter-hunk-context=-1")

How do we feel about not doing the fetch, and doing a git diff between R:/path/to/S and the cleanly generated T, instead of git diff-tree.