fix(view): indent a flat thread's replies like any other

A reply in a thread with no usable In-Reply-To carries depth 0, because that is
how notmuch reports every message of such a thread. CardLayout read depth 0 as
"not nested", so those replies drew flush against their own thread with no
spine, while a nested thread's replies indented normally: the list showed two
different shapes for the same relationship, side by side.

A MESSAGE row is nested at least one level whatever depth it reports. Being a
child row IS the nesting; the depth only says how much further to go.

This is the third fault from the same root. The depth numbering was trusted to
mean structure when it only ever meant "how notmuch happened to thread this":
first it hid a flat thread's replies entirely, then it left the first message
unreachable, and now it drew the survivors without their indent.

0 files changed, 0 insertions, 0 deletions