[BZ#773840] Show push/pull arrows even for empty text (deletes)

Submitted by Phil Hord

Description

When a block of different lines is empty, copying the lines from that side to the other is effectively a "delete" operation. If both blocks were non-empty, two arrows would be shown allowing the user to copy the left block or the right block over to the other side. When one block is empty, though, the only obvious action offered by the UI is to copy the non-empty block onto the empty one.

The user can press Shift to change the error to X (delete), but this is the same as is offered when both blocks are non-empty. Also, it's not an ideal workaround to this issue.

An arrow should be shown on the gutter of the "empty" block to allow it to replace the non-empty match on the other side. Previously, the UI acted like such a button exists already. If you would click on the flat-line of the empty block where a button would appear (in the gutter) then it would delete the block on the other side. But this behavior has been removed, probably as some bug fix.

This functionality would be correct if *) a button were visible to let the user know this would happen and *) the clickable region was {button-height} pixels tall instead of just 2 or three pixels tall.