feat: copy fixup message

[hugojosefson]

Add a feature to the right-click menu of a commit, "Copy fixup message".

Takes the first line of the selected commit's message, prepends fixup! unless it's already there, then copies the line to clipboard.

Useful for when you plan on adding things to a specific commit; you pick up the fixup string from gitk on the commit you want to add to, then paste in whatever program you are creating new commits.

• https://www.tcl.tk/man/tcl8.4/TkCmd/clipboard.htm
• https://x.org/releases/X11R7.6/doc/xorg-docs/specs/ICCCM/icccm.html#target_atoms (TEXT?)