Set PRIMARY selection on clicking a commit in the list

[jwhitaker-swiftnav]

Problem description

Copying hashes is a little bit clunky, see #716 and #580 .

Preferred solution

An alternative/additional approach would be, when the user selects a commit in the commit list, to set the PRIMARY X11/wayland selection to the commit hash. This is analogous to what happens when you select text generally so is IMO unsurprising behaviour, even though it manages to copy the commit hash with no explicit action from the user at all. The user can then use the middle-mouse button to paste it elsewhere.

Drawbacks

MacOS and Windows lack the concept of a PRIMARY selection so this would only be a decent approach on Linux afaik.