fix(mr): use standard unified diff headers in mr diff

Description

This MR fixes the default output format of glab mr diff. Currently, the command artificially reconstructs a minimal unified diff by stitching together --- and +++ file paths. While this hides noisy Git index hashes, omitting the standard diff --git separator and file mode headers breaks massive ecosystem compatibility with standard patch tools, syntax highlighters, and AI agents.

This change adds the standard diff --git separator, a/b path prefixes, file mode lines for added/deleted files, and rename from/to headers back into the default view. It maintains the project's intended "minimal" aesthetic while instantly making the output compliant with the universal standard so that tools like git apply can process it flawlessly.

Breaking change: glab mr diff output now uses a//b/ prefixes; scripts applying it with -p0 need -p1, or should switch to --raw.

Related Issues

Resolves #8542 (closed)

How has this been tested?

Ran go test ./internal/commands/mr/diff/... locally. All assertions still pass successfully because the existing tests (assert.Contains) check for diff body content, which is preserved perfectly by this structural header fix.

Screenshots (if appropriate):

N/A