Added `[merge]` keyword in prompt when a merge is in progress

[twolfson]

As brought up in #25, a merge can sometimes be clean which makes it seem like the prompt is git status is normal. To remedy this behavior, various avenues were explored (e.g. new symbols, alternate colors). However, one seemed to fit in well with the current prompt:

In this PR:

• Added get_git_progress to prompt which outputs [merge] if a merge is occurring
• Added get_git_progress to prompt, defaulting to red
• Added PROMPT_GIT_PROGRESS_COLOR as an available color override
• Added tests for get_git_progress
• Updated prompt tests to reflect get_git_progress's addition

@rpdelaney Can you please look over the changes and approve/disapprove? Following this PR's merge, we can follow up with another one that adds all missing states (e.g. rebase, cherry-pick) from the git code referenced in #25.

[rpdelaney]

Sure, happy to. Please assign this to me so I don't lose track.

[twolfson]

Added you as a collaborator and assigned to you. Please don't push any changes to master x_x

[rpdelaney]

Ahh I didn't know I had to be a collaborator to be assigned a task. I have a fork so I'll continue to send any pull requests from there.

[twolfson]

Any progress on being able to review this?

[rpdelaney]

Sorry, I've had my hands full the past couple weeks. I'll have a look at this today.

[rpdelaney]

I have no other comments. :white_check_mark:

[twolfson]

Cool, I will adjust and land this tonight =)

[twolfson]

There are actually a lot more unquoted variables in prompt_test than I would like. I am going to land this an follow up with a patch release for them.

[twolfson]

Alright, this has been released in 0.19.0. Follow-up PR for those unquoted items coming up.