Allow disabling the use of git for the file-comment.

 * src/compiler/main.lisp:
   * Add *FILE-COMMENT-FROM-GIT* to control whether to use git to
     derive the file-comment.  Default is T.  Otherwise, the actual
     file-comment is used.
   * Update PROCESS-FILE-COMMENT to use *FILE-COMMENT-FROM-GIT*.

 * bin/build-all.sh
 * bin/build.sh
 * bin/build-world.sh
   * Add -G option to control whether file-comment's are derived from
     git.