Improve handling of mscore/revision.h

Added a default mscore/revision.h containing "Unknown" as the
revision number (mscore/revision.h remains in .gitignore).
Removed the dependency on "revision" in the Makefiles. Now
"make revision" must be done manually if required.
Added sample post-checkout git hook to allow git automatically
to update mscore/revision.h when doing a git checkout. If installed,
this removes the need to do "make revision" manually.