Put worktree outside of repo in manual steps

[andrross]

If a backport fails then steps are given to create a backport manually. Currently, these steps put the worktree inside the repository folder, which causes git to see the worktree as untracked files. This change puts the worktree outside of the repository to avoid this problem.

Signed-off-by: Andrew Ross andrross@amazon.com

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[rursprung]

i just realised that there's a serious problem with this and would suggest to either amend it or revert the commit: i presume that most people - like me - have their OpenSearch repositories in the same folder. and all of them share the same branch names for release branches. this means, that if i'm doing manual backports based on this guide for the same releases in multiple OpenSearch repositories it'll try to use the same folder name for the worktrees of different repositories.

so the guide should probably add the repository name to the backport folder (either as a subfolder: ../.worktrees/${reponame}/backport-${base} or just as part of the folder name: ../.worktrees/backport-${reponame}-${base}).

and another problem (unrelated to this PR): the script never validates where it is and the guide doesn't explicitly say that you should run it from the base folder of the repository. if you run it from anywhere else (any subfolder within the repo) your folder will end up elsewhere. this could probably be solved using git rev-parse --show-toplevel (haven't had to use that one before, but found it online :)). when you run basename on that in turn you'll get the name of the folder of the repo (which is usually the repo name unless the developer chose another name for his local clone - but that doesn't matter: it's unique, we can use it for the worktree name as well).

[andrross]

@rursprung Created another PR here: #7