[#16] Colour staged files

[LeedsJohn]

Hi - this PR addresses issue 16 - Colour staged files in 'zbg status'.

I created a function to get a list of staged files using git diff --name-only --cached [commit]. This is used so that I could get a list of staged files before calling with_deleted_files and with_untracked_files (which temporarily stage deleted and untracked files). Then, we can pass the list of staged files along to fmt_diff_stats so we know which files we want to change to be green and bold.

There is still a loss of information in this implementation compared to calling git status - if a file is staged and then modified again, zbg status will now highlight the name green but git status would list the file as both staged and modified. Do you have any thoughts about this discrepancy?

Here's an example of the current behavior:

Let me know what changes you would like for me to make - I'm happy to rewrite if you think this problem should be solved differently. Thanks!

[chshersh]

I like the implementation! Really neat 🚀

There is still a loss of information in this implementation compared to calling git status - if a file is staged and then modified again, zbg status will now highlight the name green but git status would list the file as both staged and modified. Do you have any thoughts about this discrepancy?

I'm fine with this loss 👍🏻

I rarely want to edit a file, stage it, modify it again and commit only the staged part. So I'm fine with displaying it as staged