Outdated master branch detection doesn't work with worktrees

[Noratrieb]

When running any bootstrap command, it complains that it can't figure out whether my master branch is outdated.
Of course it does, as it just YOLOs into .git, which is just a file and not a directory for worktrees:

rust/src/tools/build_helper/src/git.rs

Line 177 in adf8d16

let branch_path = git_dir.join(".git/refs/remotes").join(&updated_master);

it should probably not do that (i don't have a suggestion on what to do instead)

regressed by #129584 @lolbinarycat @albertlarsan68

[lolbinarycat]

it should probably not do that (i don't have a suggestion on what to do instead)

i would say adding a bit of error handling that checks if it's in a worktree doesn't sound too hard.

there's two options from there:

1. never emit the warning in a worktree. this is basically trivial, but if someone does 100% of their work in a worktree, they may end up with an outdated upstream and not know it
2. fall back to a more reliable method when in a worktree