Allow using git difftool instead of git diff

[mkurz]

Is your feature request related to a problem? Please describe.

I'm always frustrated when I can not use git difftool but only git diff.

Describe the solution you'd like

I want the diffs displayed using git difftool.

Additional context

You hardcoded git diff here (maybe other places too, not sure):

yay/pkg/menus/diff_menu.go

Lines 55 to 58 in bea53a4

	args := []string{
	"diff",
	start + "..HEAD@{upstream}", "--src-prefix",
	dir + "/", "--dst-prefix", dir + "/", "--", ".", ":(exclude).SRCINFO",

Would be nice if there would be a (global) config and a yay command line flag to set/choose if to use diff or difftool.

Thanks!