fix(mr list --not-label): do not show MRs with specified label

Description

Fixes the ability to filter MRs by label using mr list --not-label "label". This feature has always been there but the NotLabel options were not being used so the --not-label was being ignored.

Related Issues

Resolves #1157 (closed)

How has this been tested?

Added test for --label and --not-label as there were previously none. Tested manually using the cli repo glab mr list --not-label "Community contribution"

Screenshots (if appropriate):

Comparison of output of glab vs UI

glab mr list --not-label "Community contribution"

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)
• Documentation
• Chore (Related to CI or Packaging to platforms)