fix update not popping back cwd

[NoahGorny]

Correctly restore old pwd in all cases of bash-it update

Description

When we update, we usually call bash-it restart, which re-execs bash. It means that the code of restoring
old pwd at the end is not being executed. I fixed this and another problem by correctly cding into to old pwd in time.

Motivation and Context

Closes #1920

How Has This Been Tested?

Locally, sadly it is very hard to automate testing of bash-it update, as it uses actual git repo and tries to pull information.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

[davidpfarrell]

I would document that _bash-it_pull_and_update_inner doesn't return when a restart happens.

Feels bit more awkward than using pushd/popd but since we're documenting the parameter and the function is only meant to be called from _bash-it_update-, this should work.

/nitipick : _bash-it_pull_and_update_inner might could be named better - It doesn't give much hint that a restart is the end action of the function and control won't return to the caller.

[NoahGorny]

Hi @davidpfarrell
I figured that while we are at it, we not improve the code just like you suggested 😄
You are welcome to take another look!