Two and three dots in difference specs

Thanks to Yarik Halchenko for this explanation.

Imagine a series of commits A, B, C, D... Imagine that there are two branches, topic and master. You branched topic off master when master was at commit 'E'. The graph of the commits looks like this:

     A---B---C topic
     /
D---E---F---G master

Then:

git diff master..topic

will output the difference from G to C (i.e. with effects of F and G), while:

git diff master...topic

would output just differences in the topic branch (i.e. only A, B, and C).