Fix little formatting bug with diff comments

The inserted placeholder needs to be surrounded with newlines. If it
doesn't sit on his own line, he destroys the normal formatting of the
diff - the format_diff method won't be happy.
Fix minor formatting problem in format_diff

Something along the following imposed problems:

+
+ def

The second line would not be highlighted properly, but only when the
pager is active.  The [^\+] part of the regex always matched the newline
following the '+' on the first line, the regex therefore matched multiple
lines and took the '+ def' with it - the reset color escape sequence
was put in right after the second line.

This is not a problem for a regular ruby puts, which flushes $stdout at
once, the highlighting can span multiple lines that way.
However with a pager like less every line gets printed on the screen
naturally - the second line had no escape sequence, it was white.

Everything's fine now.