Update revision file generation

[burgerrg]

• In version 9, the revision file has a second line naming the revision control system. This pull request adds "git" in the second line.
• The build process now properly identifies the .git file when used as a submodule.
• BUILDING is updated to reflect the unconditional inclusion of boot/pb.

[mflatt]

This generally looks good to me.

The reason for the old loop to search for .git in ancestor directories (instead of assuming ..) was to work when the ChezScheme directory is vendored into another repo, such as racket/racket. I don't feel strongly about this, though, and the racket/racket vendored ChezScheme could just have a difference here.

[burgerrg]

OK, I think the loop does the same logic that git describe uses internally, so see what you think now.

[burgerrg]

@mflatt, is it possible for c/build.zuo to catch the non-0 exit code when git describe fails and then write the literal string? That would avoid the loop entirely.

[mflatt]

@burgerrg The revised loop looks right to me.

The process-status function gets the exit code for a completed process, so that could be used to detect whether a git command succeeded. I prefer not trying to run git if the source does not appear to be in a Git repository, but I can see either choice.