Fix test race condition

[technoweenie]

This attempts to fix the race condition regarding $HOME in the integration tests.

Now, an initial $TESTHOME is setup at $REMOTEDIR/home. It has the starting git config and OSX keychain (if needed). begin_test() resets $HOME to $TRASHDIR/home, which is unique per test script. Tests run sequentially in a single script.

I also ran into an issue where begin_test() was silently failing, causing the scripts to bail after the first test. So, I added something in shutdown() that ensures every test runs by grepping the file for all begin_test() calls.

[technoweenie]

Also updated all the scripts to use #!/usr/bin/env bash. There are a few hold outs:

• rpm/SPECS/git.spec
• The Git pre-push hook that Git LFS installs uses #!/bin/sh. I'll change that in another pr.

There were a few that pass args to the shebang. I changed those to use the same standard shebang with an explicit set for the args.

[technoweenie]

Feels better, but not a complete fix :(

• https://travis-ci.org/github/git-lfs/builds/73308232

[rubyist]

Doh, this is my fault. #544

[technoweenie]

With @sinbad's help, I was able to replicate the issue that he fixed in #520. We both confirmed that this PR doesn't undo those fixes.