For consistency with the previous commit.
Note that this menu entry is used both for unstaged and for staged changes, and
for staged changes it is not quite accurate, as we are not discarding changes in
that case (just unstaging them). Not sure it's worth fixing this; it's still
better than "Delete", anyway.
The title was saying "Unstage lines", which was just wrong. The text said
"Delete lines", which can be seen as a bit misleading; we are only discarding
the changes to the selected lines, not deleting the lines themselves.
For consistency, rename the config variable skipUnstageLineWarning accordingly.
