meld:master commits

Add button "Unstage" to version control context menu See merge request !130

This is difficult to phrase in a way that isn't very Git-specific, but I feel like this is slightly clearer.

If a newly-added file has been staged, the unstage command is also the correct way to revert that change.

While reusing icons like this isn't ideal, it's better than not having anything. We need to (possibly in a GTK 4 port) add an actual version control icon set.

There's no good reason to reset to the top of the tree if we've been passed a sub-tree to run the scan on; we can just use whatever tree path we've been given.

This caused a very round-about bug where file status was not updated in tabs that came after a new comparison tab.