Set up git pre-commit hook and format the code automatically

[kt3k]

Related to #363 (but not a direct solution)

This PR sets up .git/hooks/pre-commit from //tools/setup.py and it runs //tools/format.py at git commit and adds back the change to the staging of commit. With this change, committed files should be always formatted and it prevents unformatted codes to appear in future pull requests.

[kitsonk]

@kt3k it needs to work on Windows too, right? Currently it doesn't.

[piscisaureus]

I don't want it to call "git add", because I very often don't commit all changes.

For this to be acceptable you need to run the contents from the index through the formatter, not the work tree file.

[kt3k]

This script formats all files, but it only adds staged files (because $staged is created from git diff --cached ... which only includes staged files.)

[kt3k]

I checked that this doesn't include unstaged changes (at least on my mac).

[hayd]

@kt3k what if you only add some lines i.e. git add -p

[piscisaureus]

What @hayd said is what I meant.
I do that all the time.

[kt3k]

I didn't think of that case enough... I'll try to think about the solution.

BTW lint-staged authors seem trying to solve a similar problem. ref. okonet/lint-staged#75
We might use the above tool when they shipped the above change?

[kt3k]

I've been trying to build on windows, but I'm seeing WindowsError: [Error 1314] from symlink function in util.py...

[piscisaureus]

@kt3k
On windows 10 -> enable developer mode
Older windows -> https://superuser.com/questions/104845/permission-to-make-symbolic-links-in-windows-7

[kt3k]

@piscisaureus
developer mode worked and setup.py passed on my windows 10. Thanks!

[kt3k]

@kitsonk
I tested on PowerShell env and emulated bash env (on win 10). And the pre-commit hook itself seems working on both environments (though the partial staging problem still remains):

[hayd]

I wonder if solution is to refactor lint.py to exit with status 1 if it made changes. And if lint makes changes in git commit hook then reject commit with a message (rather than git add).

[kt3k]

Thanks for the suggestions and reviews. I understand rejecting commit is safer than fixing automatically, but I'm afraid it could be very stressful for contributors because of the loss of commit messages.

Now I think this topic is too premature. I'm closing for now. Sorry for bothering..