install from checkout

ref #20 
@syohex

```perl -v``` with a blead perl shows a 7-digit revision number like this:
```
This is perl 5, version 21, subversion 5 (v5.21.5 (v5.21.4-65-g1dc0863)) built for x86_64-linux
```

The same for ```git log --abbrev-commit``` in a checkout directory:
```
commit 0451842
Author: Craig A. Berry <craigberry@mac.com>
Date:   Tue Sep 23 22:12:27 2014 -0500

    VMS does not have lc_monetary_2008 items in lconv.
```

I'm not sure if this is also true in the future, but in the meantime 10 is big enough for Perl.