Document how to disable difftastic temporarily

[greg0ire]

It can be useful if you want to obtain a diff in the standard format and copy paste it to github, for instance, like this:

diff --git a/manual/src/git.md b/manual/src/git.md
index 8700a3ee6..acafb665d 100644
--- a/manual/src/git.md
+++ b/manual/src/git.md
@@ -93,3 +93,5 @@ $ git show HEAD --ext-diff
 $ git log -p --ext-diff

[Wilfred]

Merged in 082abf6, thanks :)