Fix repo create for repo dirs other than .git

[claui]

This pull request replaces the current implementation of the isLocalRepo function in repo/create/create.go, which is based on --git-dir and wrongly assumes that if and only if the current directory is on the top level of a Git repository, then the output is a) a relative path, which b) points to the .git directory.

For an example that demonstrates why these assumptions are incorrect, see #8472.

To figure out whether we’re a work tree at all, the proposed implementation uses TopLevelDir instead of GitDir.
(Calling ToplevelDir is equivalent to git rev-parse --show-toplevel.)

It also introduces an isToplevel function, which calls PathFromRoot internally.
(Calling PathFromRoot is equivalent to git rev-parse --show-prefix.)

Fixes #8472.

[andyfeller]

@claui : thank you for contributing to the GitHub CLI and your patience! 🙇

Reviewing the code changes, I think they make sense given the context. The tests are failing because the new Git commands used weren't mocked out, which appear to be in 3 places:

• cli/pkg/cmd/repo/create/create_test.go

  Lines 439 to 442 in e524c1d

  	execStubs: func(cs *run.CommandStubber) {
  	cs.Register(`git -C . rev-parse --git-dir`, 0, ".git")
  	cs.Register(`git -C . rev-parse HEAD`, 0, "commithash")
  	},

• cli/pkg/cmd/repo/create/create_test.go

  Lines 505 to 510 in e524c1d

  	execStubs: func(cs *run.CommandStubber) {
  	cs.Register(`git -C . rev-parse --git-dir`, 0, ".git")
  	cs.Register(`git -C . rev-parse HEAD`, 0, "commithash")
  	cs.Register(`git -C . remote add origin https://github.com/OWNER/REPO`, 0, "")
  	cs.Register(`git -C . push --set-upstream origin HEAD`, 0, "")
  	},

• cli/pkg/cmd/repo/create/create_test.go

  Lines 691 to 695 in e524c1d

  	execStubs: func(cs *run.CommandStubber) {
  	cs.Register(`git -C . rev-parse --git-dir`, 0, ".git")
  	cs.Register(`git -C . rev-parse HEAD`, 0, "commithash")
  	cs.Register(`git -C . remote add origin https://github.com/OWNER/REPO`, 0, "")
  	},

Additionally since this scenario is focused on non-standard .git directories, what do you think about creating a new test case where we specifically ensure that something like .git2 is matched as expected?

[williammartin]

Gentle nudge on this review @claui

[williammartin]

This PR has gone stale so I'm going to close it. I would welcome anyone who wants to pick it up to start from here and address the review.