bug#41604: guix pull impossible after rebasing a local repository

[John Soo]

Hi Arne,

Interesting idea about mercurial.

My problem largely comes from the fact that I specified a branch in 
channels.scm for over a year with the same workflow. If a branch can be 
specified for a channel repo then the system probably should handle the current 
pull commit not being available in the history.

The commit history authentication is nice but seems to have regressed this 
particular use case. 

- John