fast-import: Hide the pack boundary commits by default.

fast-import: Hide the pack boundary commits by default.

Most users don't need the pack boundary information that fast-import
was printing to standard output, especially if they were calling
it with --quiet.

Those users who do want this information probably want it captured
so they can go back and use it to repack the imported repository.
So dumping the boundary commits to a log file makes more sense then
printing them to standard output.

Signed-off-by: Shawn O. Pearce <redacted>