diff: Fix --expand-tabs and --side-by-side.

[des]

Summary

• Overhaul column width and padding calculation.
• Rewrite print_space() so it is now a) correct and b) understandable.
• Rewrite tab expansion in fetch() for the same reason.

This brings us in line with GNU diff for all cases I could think of.

Sponsored by: Klara, Inc.

[des]

fix tab expansion edge case

[markj]

usr.bin/diff/diffreg.c

255

The padding is always one space or one tab, depending on whether --expand-tab is set?

267

Do I understand correctly that hw == 0 just means that there's no padding around the change marker?

277

Presumably we should also set hw = 0 if width < 3?

[des]

usr.bin/diff/diffreg.c

255

No, the padding is variable and print_space() will use tabs and / or spaces as appropriate to reach the correct position.

267

No, hw == 0 means that either we're not in side-by-side mode or we are but the width was set so narrow that there's no room to display anything other than the marker and possibly padding.

277

you're right, --tabsize=1 --width=2 is not covered, I'll update.

[des]

cover degenerate case where tabsize <= width <= 3

[des]

<=