create-pull-request: cd to relative directory

create-pull-request -d path creates empty patches if directory
is specified as a path, i.e. ./bitbake or ./bitbake/ or full path.
It behaves expected way only if script is run with -d bitbake, i.e.
relative dir name doesn't contain '\'.

Fixed this unwanted behaviour by changing directory and running
git format-patch in it with --relative, without specifying
relative path as a parameter.

(From OE-Core rev: 3042956a86167f89beccc5d05f1fad1844e7c36a)

Signed-off-by: Ed Bartosh <ed.bartosh@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>