[PATCH] branch: implement shortcut to delete last branch

With the approvals listed in [*1*] and in accordance with the
guidelines set out in Documentation/SubmittingPatches, I am submitting
this patch to be applied upstream.

After work on this patch is done, I'll look into picking up where the
prior work done in [*2*] left off.

Is there anything else that needs to be done before this can be
accepted?

[Reference]

*1* https://public-inbox.org/git/1521844835-23956-2-git-send-email-p@aaronjgreenberg.com/
*2* https://public-inbox.org/git/1488007487-12965-1-git-send-email-kannan.siddharth12@gmail.com/