This commit should fix "move con to parent" trick (see below) in the
case when con->parent->parent is a workspace.
The trick:
mark _a, focus parent, focus parent, mark _b,
[con_mark=_a] move window to mark _b, [con_mark=_a] focus
The trick got broken in commit 626af81232
in order to fix an i3 crash (#2003). Reverting said commit fixes the
trick. The crash is caused by the fact that empty workspace isn't
considered a split (checked in src/con.c:1324), so the moved window ends
up as a sibling of the target workspace, not as its child.
beb96ad18c