feat(mr): support minimal unified diff output for mr diff

Description

This merge request modifies the output of glab mr diff to have valid unified diff headers for a set of diff hunks. This provides a more predictable output format regardless of the user's pager or diff viewer. It does not provide the full header output that git diff provides.

This also adds a small blurb in the help for glab config that describes being able to set a custom pager via glab_pager.

Related Issue

Related to #[1016] although it may not completely resolve it

How Has This Been Tested?

Tested via both make test and manually using delta as a custom pager set with glab config set glab_pager delta -g and checking several different glab mr outputs.

Screenshots (if appropriate):

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)