Don't put VCS details in Constants.scala when not needed

This is useful when developing on any of the stubs modules (stubs,
runner, test-runner), but this triggers a recompilation of build very
often. This is now disabled by default. When developing on the stubs
modules, set `local-repo`.developingOnStubModules to true in build.sc,
for Mill's watch mode to take into account changes in them.

Author: alexarchambault

build.sc

@@ -233,13 +233,16 @@ class Build(val crossScalaVersion: String)
         """ !sv.startsWith("2.") """
       else
         "false"
+    val detailedVersionValue =
+      if (`local-repo`.developingOnStubModules) s"""Some("${vcsState()}")"""
+      else "None"
     val code =
       s"""package scala.build.internal
          |
          |/** Build-time constants. Generated by mill. */
          |object Constants {
          |  def version = "${publishVersion()}"
-         |  def detailedVersion = "${vcsState()}"
+         |  def detailedVersion: Option[String] = $detailedVersionValue
          |
          |  def scalaJsVersion = "${Deps.scalaJsLinker.dep.version}"
          |  def scalaNativeVersion = "${Deps.nativeTools.dep.version}"
@@ -613,6 +616,14 @@ class TastyLib(val crossScalaVersion: String) extends ScalaCliCrossSbtModule
 }
 
 object `local-repo` extends LocalRepo {
+
+  /*
+   * If you are developing locally on any of the stub modules (stubs, runner, test-runner),
+   * set this to true, so that Mill's watch mode takes into account changes in those modules
+   * when embedding their JARs in the Scala CLI launcher.
+   */
+  def developingOnStubModules = false
+
   def stubsModules = {
     val javaModules = Seq(
       stubs

modules/cli/src/main/scala/scala/cli/commands/About.scala

@@ -10,7 +10,7 @@ class About(isSipScala: Boolean) extends ScalaCommand[AboutOptions] {
   def run(options: AboutOptions, args: RemainingArgs): Unit = {
     CurrentParams.verbosity = options.verbosity.verbosity
     val version            = Constants.version
-    val detailedVersionOpt = Some(Constants.detailedVersion).filter(_ != version)
+    val detailedVersionOpt = Constants.detailedVersion.filter(_ != version)
     val appName =
       if (isSipScala) "Scala command"
       else "Scala CLI"