west update: generalize -k / -r to include commits, not just branches

This is a behavioral change originally suggested in #380, so I’m opening an issue to solicit feedback.

Currently, west update --keep-descendants and west update --rebase only work if there is a branch checked out. They ignore detached heads and do not try to keep or rebase them.

This issue tracks generalizing their behavior to also work when HEAD is detached. We could have a configuration option to keep the old behavior.

cc: @carlescufi @tejlmand @marc-hb

[marc-hb]

Having lost detached HEADs many times when using grepo, I couldn’t agree more. grepo’s “solution” to this problem is (was?) “don’t forget to create branches”. This is silly: we use computers precisely because they don’t forget what we do.

The reason users regularly “forget to create branches” is super obvious: it’s simply because plain git uses branches by default whereas grepo and west do not use branches by default. Inconsistent usability - for valid reason(s) I’m sure but inconsistent usability still.

I can think of a possibly simpler/cheaper middle ground between 1. #381 automagically rebasing everything even detached HEADs and 3. #382 just warn when something bad happens: 2. Refuse to west rebase if any detached HEAD is found. This could be opt-in in ~/.westconfig

By the way I wonder whether west more generally supports the concept of a transactional west update. Last time I tried it did not. A plain git rebase or merge is either successful or it’s not, I mean it either commits all the files or none. Plain git also has the incredibly useful --abort feature. Does west have the ability to “successfully west rebase all repos or none”? Part of why we have manifests is to control dependencies across repos. #338 is related.

[marc-hb]

Mere bidirectional link to related https://github.com/zephyrproject-rtos/zephyr/pull/31201 doc: west: add note about west update --keep-descendants option #31201