Optimize diff-hl--update

[jamescherti]

This pull request optimizes diff-hl--update.

[jamescherti]

This adds two optimizations during the buffer traversal.

First, (eq buf base) is checked before calling (buffer-base-buffer buf). This short-circuits evaluation to bypass the buffer-base-buffer call overhead.

Second, the boolean state of (or changes ref-changes) outside the loop. This prevents the Lisp interpreter from recalculating the same static condition for all active buffers.