version: add a manual date as version for now

Maybe we'll auto-bump this with a bot over time.

See golang/go#37475 & golang/go#29814

Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

version/version.go

@@ -7,5 +7,5 @@
 // Package version provides the version that the binary was built at.
 package version
 
-const LONG = "LONGVER-TODO"
-const SHORT = "SHORTVER-TODO"
+const LONG = "date.20200228"
+const SHORT = LONG // TODO: unused; remove SHORT? Make it a func?