isSnapshot may be false for manually overriden versions

When using `sbt -Dscala.version=2.X.Y` to test out packages,
newer version of sbt-git have been setting `isSnapshot` to false
if there isn't a tag associated with the current commit of
scala/scala-dist. This makes it hard to test out changes in
this repo as we get diffs like:

```
$ rpm -qip /home/ubuntu/code/scala-dist-baseline/target/scala-2.13.6.rpm
Name        : scala
Version     : 2.13.6
Release     : 1
Architecture: noarch
Install Date: (not installed)
Group       : Development/Languages
Size        : 649267584
License     : BSD
Signature   : (none)
Source RPM  : scala-2.13.6-1.src.rpm
```

```
$ rpm -qip  /home/ubuntu/code/scala-dist/target/scala-2.13.6.rpm
Name        : scala
Version     : 2.13.6
Release     : SNAPSHOT
Architecture: noarch
Install Date: (not installed)
Group       : Development/Languages
Size        : 649267584
License     : BSD
Signature   : (none)
Source RPM  : scala-2.13.6-SNAPSHOT.src.rpm
...
```

Author: retronym

build.sbt

@@ -8,6 +8,13 @@ useJGit
 // For testing, the version may be overridden with -Dproject.version=...
 versionWithGit
 
+isSnapshot := {
+  git.overrideVersion(git.versionProperty.value) match {
+    case Some(v) => v.endsWith("-SNAPSHOT") || git.gitUncommittedChanges.value
+    case _ => isSnapshot.value // defined in SbtGit.scala
+  }
+}
+
 Versioning.settings
 
 // necessary since sbt 0.13.12 for some dark and mysterious reason