Squelch unneeded and misleading warnings from "repack" when the
command attempts to generate pack bitmaps without explicitly asked
for by the user.
* jk/repack-silence-auto-bitmap-warning:
repack: simplify handling of auto-bitmaps and .keep files
repack: silence warnings when auto-enabled bitmaps cannot be built
t7700: clean up .keep file in bitmap-writing test