What does a/ and b/ stands for in patch header?
A' and the 'B' are prefixes to differentiate source and destination.

You can change the prefixes with 

--src-prefix=&lt;prefix&gt;
Show the given source prefix instead of "a/".

--dst-prefix=&lt;prefix&gt;
Show the given destination prefix instead of "b/".


From http://git-scm.com/docs/git-format-patch