Add a user config for using git's external diff command for paging

This is similar to using lazygit's Git.Paging.ExternalDiffCommand config, except
that the command is configured in git. This can be done either with git's
`diff.external` config, or through .gitattributes, so it gives a bit more
flexibility.

docs/Config.md

@@ -306,6 +306,9 @@ git:
     # e.g. 'difft --color=always'
     externalDiffCommand: ""
 
+    # If true, Lazygit will use git's `diff.external` config for paging. The advantage over `externalDiffCommand` is that this can be configured per file type in .gitattributes; see https://git-scm.com/docs/gitattributes#_defining_an_external_diff_driver.
+    useExternalDiffGitConfig: false
+
   # Config relating to committing
   commit:
     # If true, pass '--signoff' flag when committing