1.0.8.15: making SBCL sources Git more friendly

 * Git for SBCL Hackers guide.

 * If branch-version.lisp-expr exists, it's value is concatenated
   after the contents of version.lisp-expr, separated out by ".".

   Doing this allows reduces spurious conflicts on branches: otherwise
   a branch announcing it's name in version.lisp-expr is going to
   conflict at every turn, which prevents automatic rebasing and
   merging from doing TRT.

 * Added .gitignore