change from spack_commit to spack_branch ignored

[adrienbernede]

If uberenv is set to clone a specific commit of spack, moving to a branch will cause the new ref to be fetch but not checked out.

So removing spack_commit: ... and optionally adding spack_branch: ... in an instance with spack already cloned will have no effect.

That’s because the implementation will only use the branch in the clone operation, which is a fetch if the clone already exists.

There is a repo_pull option to force update of a spack branch each time uberenv is called. This option is not documented. It seems weird to have to setup an option fetch a branch.

Here is what I suggest:

• OK, using a branch to point to spack is unstable, and not desirable in production.
• but pointing to a branch should imply keeping it up-to-date, at least by default, this would allow to switch from one branch to another, and fix this issue.
• repo_pull should therefore be changed to do_not_pull, so that it is explicit in the configuration that we enforce one particular ref