Check if `.git/hooks/pre-commit` file exists

[lcidral]

As reported by @amadeu01, there is a possibility of developer has .git/hooks/pre-commit, if so we can't overwrite it, returning an error when running git pair init.

We also, discussed to introduce a script to create a pre-commit file that would iterate over .git/hooks/pre-commits directory then call it script under it (including our git-pair _append-coauthors call). But I think that we should create a separate issue for this one. WDYT?

[amadeu01]

Handling more hooks per trigger seems to be another task outside the scope of the project.
I think if some user already has a hook for pre-commit event trigger, we should let them call the CLI without the hooker.

[amadeu01]

(We can make another tool for handling more hooks scripts per events )

[wevtimoteo]

I think we can, initially, consider a separate task for it.