[cherry-picker] Misleading assumption about the main branch

[webknjaz]

Hi,

This is probably a follow-up for #225.

On some projects I work on, the repo's default branch is not master, but devel or smth else. It would be nice to have it configurable and maybe query this information from github repo metadata as a fallback.

[terryjreedy]

AFAIK, the only hardcoded use of 'master' is in 'git checkout master' at cleanup. Since I want to run cherry_picker in the worktree for target branch, I edited 'master' to, then, '3.6'. Now that there are both 3.6 and 3.7 as backport targets, even this is problemmatical.

Several months ago, I requested that cherry_picker stop assuming that it is being run in the master repository, and instead cleanup by checking out the branch that was originally checked out in the worktree. This is each to determine with 'git branch'. The current branch is prefixed with '* '. This could be detected on startup. This would also solve the problem of a master branch nameed something else.

[serhiy-storchaka]

git symbolic-ref --short HEAD gives you the name of the current branch.

[webknjaz]

Fair enough. I'll try to update this in #254

[webknjaz]

@terryjreedy @serhiy-storchaka can someone review the PRs, please?