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/version.go

@@ -7,5 +7,35 @@
 // Package version provides the version that the binary was built at.
 package version
 
-const LONG = "date.20200921"
-const SHORT = LONG
+// Long is a full version number for this build, of the form
+// "x.y.z-commithash", or "date.yyyymmdd" if no actual version was
+// provided.
+const Long = "date.20200921"
+
+// Short is a short version number for this build, of the form
+// "x.y.z", or "date.yyyymmdd" if no actual version was provided.
+const Short = Long
+
+// LONG is a deprecated alias for Long. Don't use it.
+const LONG = Long
+
+// SHORT is a deprecated alias for Short. Don't use it.
+const SHORT = Short
+
+// GitCommit, if non-empty, is the git commit of the
+// github.com/tailscale/tailscale repository at which Tailscale was
+// built. Its format is the one returned by `git describe --always
+// --exclude "*" --dirty --abbrev=200`.
+const GitCommit = ""
+
+// ExtraGitCommit, if non-empty, is the git commit of a "supplemental"
+// repository at which Tailscale was built. Its format is the same as
+// GitRevision.
+//
+// ExtraGitRevision is used to track the source revision when the main
+// Tailscale repository is integrated into and built from another
+// repository (for example, Tailscale's proprietary code, or the
+// Android OSS repository). Together, GitRevision and ExtraGitRevision
+// exactly describe what repositories and commits were used in a
+// build.
+const ExtraGitCommit = ""