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>

build_dist.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env sh
+#
+# Runs `go build` with flags configured for binary distribution. All
+# it does differently from `go build` is burn git commit and version
+# information into the binaries, so that we can track down user
+# issues.
+#
+# If you're packaging Tailscale for a distro, please consider using
+# this script, or executing equivalent commands in your
+# distro-specific build system.
+
+set -euo pipefail
+
+describe=$(git describe --long --abbrev=9)
+# --abbrev=200 is an arbitrary large number to capture the entire git
+# hash without trying to compact it.
+commit=$(git describe --dirty --exclude "*" --always --abbrev=200)
+
+long=$(./version/mkversion.sh long "$describe" "")
+short=$(./version/mkversion.sh short "$describe" "")
+
+exec go build -tags xversion -ldflags "-X tailscale.com/version.Long=${long} -X tailscale.com/version.Short=${short} -X tailscale.com/version.GitCommit=${commit}" "$@"