A git-only SBCL workflow

A git-only SBCL workflow

 This updates the SBCL build and release process to be more compatible
 with distributed development -- to facilitate moving the upstream
 repository into Git.

 A detailed description of what is going on here is in
 doc/GIT-WORKFLOW.md.

 Some highlights:

  * Drop version.lisp-expr and branch-version.lisp-expr.

  * Auto-generate the version at build time using information
    from Git, incorporating:
    - Last release number.
    - Number of commits on origin/master since last release.
    - Current branch, if there are commits not on origin/master.
    - Number of commits not on origin/master.
    - SHA1 id of the last commit.
    - Optional -dirty marker.

  * Update release.sh to work with Git.

  * Make source-distribution.sh exclude the .git directory from tarballs.

  * Release tags contain NEWS for that release.