fix: Handle cons cell value for line-spacing

The `line-spacing` variable can be a [cons cell][1], 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. This change uses `total-line-spacing` instead.

[1]: emacs-mirror/emacs@e8f26d5

Author: prashantvithani

diff-hl.el

@@ -314,7 +314,7 @@ It can be a relative expression as well, such as \"HEAD^\" with Git, or
                          (numberp text-scale-mode-amount))
                     (expt text-scale-mode-step text-scale-mode-amount)
                   1))
-         (spacing (or (and (display-graphic-p) (default-value 'line-spacing)) 0))
+         (spacing (or (and (display-graphic-p) (total-line-spacing (default-value 'line-spacing))) 0))
          (h (+ (ceiling (* (frame-char-height) scale))
                (if (floatp spacing)
                    (truncate (* (frame-char-height) spacing))