diff: fix side-by-side output with tabbed input

The previous logic conflated some things... in this block:

• j: input characters rendered so far
• nc: number of characters in the line
• col: columns rendered so far
• hw: column width ((h)ard (w)idth?)

Comparing j to hw or col to nc are naturally wrong, as col and hw are
limits on their respective counters and nc is already brought down to hw
if the input line should be truncated to start with.

Right now, we end up easily truncating lines with tabs in them as we
count each tab for $tabwidth lines in the input line, but we really
should only be accounting for them in the column count. The problem is
most easily demonstrated by the two input files added for the tests,
the two tabbed lines lose at least a word or two even though there's
plenty of space left in the row for each side.

Reviewed by: bapt, pstef
Sponsored by: Klara, Inc.
Differential Revision: https://reviews.freebsd.org/D37676