install from checkout

ref #20 
Is 10 digits enough to identify ? Should it be longer than 10 digits ?
Or should all of SHA1 be used rather than substring ?

10 digits of SHA1 may have collision with other SHA1 in large project, such as Linux kernel.
I suppose Perl is large project. (However I don't know how large Perl project is.)

Reference
http://git-scm.com/book/ch6-1.html#Short-SHA