cygport suggestion: Make "prepare" an alias for "prep"

[Hamish McIntyre-Bhatty]

Hi there,

Admittedly a very minor issue, but I thought I should at least mention it.

Most of cygport's commands aren't abbreviated, but prepare is 
abbreviated to "prep". It'd be nice if "prepare" was accepted too for 
consistency.

Thanks for all the work put into this tool, I can't imagine being a 
cygwin package maintainer without it.

Hamish