version: use OSS repo's version when building.

When building with redo, also include the git commit hash
from the proprietary repo, so that we have a precise commit
that identifies all build info (including Go toolchain version).

Add a top-level build script demonstrating to downstream distros
how to burn the right information into builds.

Adjust `tailscale version` to print commit hashes when available.

Fixes #841.

Signed-off-by: David Anderson <danderson@tailscale.com>

version/describe.txt.do

@@ -1,4 +1,4 @@
-describe=$(cd ../.. && git describe --long --abbrev=9)
+describe=$(git describe --long --abbrev=9)
 echo "$describe" >$3
 redo-always
 redo-stamp <$3