Don't use "<unknown>" for placeholders and suppress printing of empty user formats.

Don't use "<unknown>" for placeholders and suppress printing of empty user formats.

This changes the interporate() to replace entries with NULL values
by the empty string, and uses it to interpolate missing fields in
custom format output used in git-log and friends.  It is most useful
to avoid <unknown> output from %b format for a commit log message
that lack any body text.

Signed-off-by: Junio C Hamano <gitster@pobox.com>