feat: add env var OBSIDIAN_GIT for scripting

[zanets]

I have a pre-push hook that ask for user confirmation. Sadly it makes obsidian-git push failed by timeout since I cannot make tty input.

A simple solution is to add an environment variable when spawn git process, check it in the hook and bypass the confirmation.

I also think configurable push command will be cool, but it's too much for this case.

[denolehov]

lgtm, @Vinzent03 agree?

[Vinzent03]

So far I didn't work on other issues, because I didn't want to get merge conflicts with #66 , but I am already waiting a long time for the missing method from the Obsidian devs. I have no clue, when they will add that. I am okay with merging this. Will implement #70 and #68 hopefully this evening and will publish a release then.