git-gui: Properly set the state of "Stage/Unstage Hunk" action

git-gui: Properly set the state of "Stage/Unstage Hunk" action

Today I found yet another way for the "Stage Hunk" and "Unstage
Hunk" context menu actions to leave the wrong state enabled in
the UI.  The problem this time was that I connected the state
determination to the value of $::current_diff_side (the side the
diff is from).  When the user was last looking at a diff from the
index side and unstages everything the diff panel goes empty, but
the action stayed enabled as we always assumed unstaging was a
valid action.

This change moves the logic for determining when the action is
enabled away from the individual side selection, as they really
are two unrelated concepts.

Signed-off-by: Shawn O. Pearce <spearce@spearce.org>