Fix usage output

The leading newline in the option string causes "git rev-parse
--parseopt" to output:

	usage:
	   or: git integration ...

when clearly we want:

	usage: git integration ...
	   or: git integration ,,,

Fix this while maintaining a readable option specification by switching
to DQs and escaping the first newline.

Signed-off-by: John Keeping <john@keeping.me.uk>