Reduce amount of git processes

[lgeiger]

Description

This PR reduces the amount of git processes required to populate the prompt from 5 to 3.

Motivation and Context

Currently querying the git status launches 5 git processes. This can be quite slow especially on larger repos. This PR removes the need to call git rev-list --count --left-right @{upstream}...HEAD and instead reads the necessary information from git --no-optional-locks status --porcelain --branch directly. I also moved the fallback code such that we do not need to call git branch --show-current in the default case where we're on a normal branch. This reduces the number of spawned git processes to 3:

main:

git branch --show-current
git rev-parse --git-dir --is-inside-git-dir
git --no-optional-locks status --porcelain
git stash list
git rev-list --count --left-right @{upstream}...HEAD

This PR:

git rev-parse --git-dir --is-inside-git-dir
git --no-optional-locks status --porcelain --branch
git stash list

Best to review commit by commit. Let me know if you prefer to split the commits into separate PRs

How Has This Been Tested

• I have tested using Linux.
• I have tested using MacOS.

Checklist

• I am ready to update the wiki accordingly.
• I have updated the tests accordingly.