[PATCH] add short options to show-files

The show-files long options are cumbersome to type.  This patch adds
equivalent short options.

Also add missing "unmerged" to usage string.

Finally reduce the number of lines for argument parsing in half.

Signed-off-by: Nicolas Pitre <nico@cam.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>