Handle range comments in Unified. In general this works the same
way as in SideBySide. The interesting cases come up when the
user's selection spans multiple diff regions that are
impossible to combine in SideBySide. For example, the selection
can span a deletion chunk and an insertion chunk. In these
cases, we forcibly move the start line to the corresponding line
on the other side as if it were displayed on SideBySide. This is
a natural "fix" to the lousy selection made by the user and
probably reflects what the user means.
Note that range comments spanning multiple regions might cause
text that aren't actually part of the comment range to be
highlighted. For example, if the user puts a range comment that
spans the entire file, then every line will be highlighted in
Unified, even though the deletion chunks are technically not part
of the comment range. While we might be able to refine the
highlighting, it will need a more complicated implementation
that doesn't seem to worth the trouble.
Change-Id: I8ec6ee3f710a31085ebe8c3f28414d76d498286c
067a7e8dc4