version: explicitly use 9 hex digits in git describe version number

So it doesn't vary based on who's doing the release with which version
of git.

Fixes tailscale/corp#419

version/describe.txt.do

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