[ET Trac] #2965: GetComponents option to obtain new branch

[Jordan Nicoules]

#2965: GetComponents option to obtain new branch

 Reporter: Jordan Nicoules
   Status: new
Milestone: 
  Version: 
     Type: enhancement
 Priority: minor
Component: GetComponents

## Context

Typically on a new release, calling `GetComponents`, even with the update option, will check out new components, but existing ones will not deal with the `!REPO_BRANCH = $ET_RELEASE` which changes in the thornlist.
On the other hand, a change in the repo's URL is detected and triggers some error, asking for a clean checkout (at least I could see that for `Carpet` and `POWER`, between ET_2023_05 and ET_2026_05).

>From a naive user perspective, I expected that updating components with a thornlist that indicates an ET version would put the repos on the corresponding branch, ready to be compiled.

## Suggested feature (details TBD)

**Either with the update option or a new option, add the `!REPO_BRANCH` to the fetch list, create the corresponding branch, and check it out if possible.**
Possibly (as an option), it could also remove the previous one.
This makes the most sense for branches that are release versions, but may not in general.

A minima, if there are possible conflicts (unstaged changes etc.), it could raise a warning/error like for the URL, that the user would then deal with manually.


## Discussion

1. Is this feature desirable? Does it correspond better to a user's expectation?
2. Can it be harmful?
3. Does it actually save work and effort, compared to recreating a brand new Cactus directory? 

About point 2, a conservative addition of a fetched ref and corresponding local branch, even if not checked out, looks benign to me.

Regarding point 3, to me one of the issues is the following. With a new Cactus directory, it's needed to manually move and redo possible local changes and/or additions (tracking more branches/remotes, local commits,...) -- even for forked/owned repositories, although it's less critical in that case.
This manual adjustment may not disappear completely, but could be alleviated (and be more explicit).

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2965/getcomponents-option-to-obtain-new-branch
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