Feature Request: Multi-base diff highlighting (show branch changes + local changes simultaneously)

[raggesilver]

Hi! I'd love to see multi-base diff functionality in gitsigns, inspired by IDE merge request workflows.

The concept

Show changes against two different bases simultaneously with distinct colors:

• Standard colors (green/yellow/red): Local uncommitted changes vs HEAD
• Additional color (e.g., purple): All branch changes vs merge-base with target branch

Real-world example

PyCharm's GitLab plugin does this with "Review Mode" (docs, discussion) - it shows pink markers for MR changes alongside standard VCS colors for local changes.

Use case

When working on a feature branch, instantly distinguish:

1. Recent work - what you just edited (uncommitted)
2. Branch context - what you changed in this entire feature branch vs main

Possible implementation

-- Example config
base_comparisons = {
  { base = "HEAD", colors = { add = "green", change = "yellow", delete = "red" }},
  { base = "origin/main", colors = { add = "purple", change = "purple", delete = "purple" }}
}

This would work for any Git workflow (GitHub PRs, GitLab MRs, or just feature branches) and help with code review, branch awareness, and understanding change context.

Questions

1. Is this technically feasible with gitsigns' architecture?
2. Any interest in this kind of multi-base comparison feature?