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>

Documentation/pretty-formats.txt

@@ -132,6 +132,14 @@ The placeholders are:
 - '%n': newline
 - '%x00': print a byte from a hex code
 
+If you add a `{plus}` (plus sign) after '%' of a placeholder, a line-feed
+is inserted immediately before the expansion if and only if the
+placeholder expands to a non-empty string.
+
+If you add a `-` (minus sign) after '%' of a placeholder, line-feeds that
+immediately precede the expansion are deleted if and only if the
+placeholder expands to an empty string.
+
 * 'tformat:'
 +
 The 'tformat:' format works exactly like 'format:', except that it