fix: Handle cons cell value for line-spacing

The `line-spacing` variable can be a [cons cell][1] from Emacs 31, such
as `(0.1 . 0.1)`, in addition to a plain number. Each value in this cons
is spacing above and below the line. The correct value of line-spacing
would be to use [`total-line-spacing`][1] instead of raw value.

The previous code did not handle the cons cell format, leading to a
`wrong-type-argument` error. For backward compatibility, this change
inlines the definition of `total-line-spacing` i.e. sums the above and
below spacing in case if it's cons.

[1]: emacs-mirror/emacs@e8f26d5

Author: prashantvithani

diff-hl.el

@@ -315,10 +315,13 @@ It can be a relative expression as well, such as \"HEAD^\" with Git, or
                     (expt text-scale-mode-step text-scale-mode-amount)
                   1))
          (spacing (or (and (display-graphic-p) (default-value 'line-spacing)) 0))
+         (total-spacing (pcase spacing
+                          ((pred numberp) spacing)
+                          (`(,above . ,below) (+ above below))))
          (h (+ (ceiling (* (frame-char-height) scale))
-               (if (floatp spacing)
-                   (truncate (* (frame-char-height) spacing))
-                 spacing)))
+               (if (floatp total-spacing)
+                   (truncate (* (frame-char-height) total-spacing))
+                 total-spacing)))
          (w (min (frame-parameter nil (intern (format "%s-fringe" diff-hl-side)))
                  diff-hl-bmp-max-width))
          (_ (when (zerop w) (setq w diff-hl-bmp-max-width)))