Allow editing the message of a stash

[FichteFoll]

Problem description

It is currently not possible to edit the message of a stash (commit). SM allows entering edit mode for a stash commit but when trying to submit the changes, it complains about the commit not being reachable from HEAD.

Preferred solution

Trying to edit the message of any stash functions exactly like editing a commit's message that is "reachable from HEAD".

Screenshots