Add GJF code style pre commit hook support.

[raviagarwal7]

Runs only on changed java files

[artem-zinnatullin]

Have you checked if it works with -a?

ie:

# Do some changes.
$ git commit -am 'commit message'

I was doing pretty much same thing to autoformat code, but faced problem with committing with -a flag.

I don't remember the details exactly, but I think -a creates secondary temporary .git/index and if you try to add something to git index in the hook, it breaks its internal state and both index files remain and git starts erroring

[raviagarwal7]

@artem-zinnatullin, verified that git commit -am 'commit message' commits with formatted changes.