Fix #33913: Unable to set sequencer proxy custom directory as relative path

Added absolute->relative conversion for directory in file_browse_exec.
This is how other places deals with this, but perhaps we'd better change
file selection so both filepath and directory are affected by relative
option.

git-svn-id: https://svn.blender.org/svnroot/bf-blender/trunk/blender@53890 954f8c5b-7b00-dc11-b283-0030488c597c