fix(bulk-model-sync-gradle): fix MPS Runner configuration for server sources with base revision

When a base revision was specified, the specified branch was not passed as a system property to the bulk sync.

When a base revision was specified without a source revision, the property value for the source revision was the string `"null"`.

To write test with TestKit we would need further adjustments to the build setup.
For this fix, I considered them out of scope.

I tried to test execution with Gradles TestKit first.
See https://docs.gradle.org/current/userguide/test_kit.html
But the Plugin tries to download the package "org.modelix:bulk-model-sync-mps" as `mpsDependencies`.
The names of the downloaded files are then used as `classPathElements` in the MPS Runner configuration.
Therefore, the plugin cannot be tested with TestKit without publishing "org.modelix:bulk-model-sync-mps" and its dependents locally or including the "modelix.core" project from within a test execution.

Author: Oleksandr Dzhychko

bulk-model-sync-gradle/src/main/kotlin/org/modelix/model/sync/bulk/gradle/MpsRunnerConfiguration.kt

@@ -81,8 +81,13 @@ internal fun buildMpsRunConfigurationForLocalTarget(
             if (source is ServerSource && source.baseRevision != null) {
                 add("-Dmodelix.mps.model.sync.bulk.server.repository=${source.repositoryId}")
                 add("-Dmodelix.mps.model.sync.bulk.server.url=${source.url}")
-                add("-Dmodelix.mps.model.sync.bulk.server.version.hash=${source.revision}")
                 add("-Dmodelix.mps.model.sync.bulk.server.version.base.hash=${source.baseRevision}")
+                if (source.branchName != null) {
+                    add("-Dmodelix.mps.model.sync.bulk.server.branch=${source.branchName}")
+                }
+                if (source.revision != null) {
+                    add("-Dmodelix.mps.model.sync.bulk.server.version.hash=${source.revision}")
+                }
             }
         },
     )

bulk-model-sync-gradle/src/test/kotlin/org/modelix/model/sync/bulk/gradle/MpsRunnerConfigurationTest.kt

@@ -59,7 +59,7 @@ class MpsRunnerConfigurationTest {
             "-Dmodelix.mps.model.sync.bulk.input.continueOnError=false",
             "-Xmx2g",
             "-Dmodelix.mps.model.sync.bulk.server.repository=aRepositoryId",
-            "-Dmodelix.mps.model.sync.bulk.server.version.hash=null",
+            "-Dmodelix.mps.model.sync.bulk.server.branch=aBranchName",
             "-Dmodelix.mps.model.sync.bulk.server.url=aUrl",
             "-Dmodelix.mps.model.sync.bulk.server.version.base.hash=aBaseRevision",
         )