Add alias for `git reset --hard` and `grh` like OMZ

[bittner]

Adds two aliases for git reset, which are inspired by OMZ.

Description

• grh is a natural acronym for git reset HEAD.
• grh! is the "dangerous" version of git reset, which throws away everything, hence the exclamation mark.

Motivation and Context

The existing Git alias for git reset is not immediately obvious, unless you are used to think in terms of "stage" and "unstage".

How Has This Been Tested?

I use this on a daily basis, having it integrated identically in a fresh installation of Bash-it on my developer laptop.

The new aliases don't clash with existing ones, as verified manually by a simple text search in the file and a grep over the repository HEAD.

Types of changes

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

Checklist:

• [x] My code follows the code style of this project.
• [ ] If my change requires a change to the documentation, I have updated the documentation accordingly.
• [x] 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.
• [x] I have added tests to cover my changes, and all the new and existing tests pass.

[bittner]

The failing of the lint job is likely unrelated to the changes in this PR. – Go error: undefined: any

[bittner]

Anyone interested to merge this PR?