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/ver.go.do

@@ -1,8 +1,12 @@
-redo-ifchange long.txt short.txt ver.go.in
+redo-ifchange long.txt short.txt gitcommit.txt extragitcommit.txt ver.go.in
 
 read -r LONGVER <long.txt
 read -r SHORTVER <short.txt
+read -r GITCOMMIT <gitcommit.txt
+read -r EXTRAGITCOMMIT <extragitcommit.txt
 
 sed -e "s/{LONGVER}/$LONGVER/g" \
     -e "s/{SHORTVER}/$SHORTVER/g" \
+    -e "s/{GITCOMMIT}/$GITCOMMIT/g" \
+    -e "s/{EXTRAGITCOMMIT}/$EXTRAGITCOMMIT/g" \
     <ver.go.in >$3