Pretty-format: %[+-]x to tweak inter-item newlines

Pretty-format: %[+-]x to tweak inter-item newlines

This teaches the "pretty" machinery to expand '%+x' to a LF followed by
the expansion of '%x' if and only if '%x' expands to a non-empty string,
and to remove LFs before '%-x' if '%x' expands to an empty string.  This
works for any supported expansion placeholder 'x'.

This is expected to be immediately useful to reproduce the commit log
message with "%s%+b%n"; "%s%n%b%n" adds one extra LF if the log message is
a one-liner.

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