Enable skipping fetching latest for BWC builds (#29497)

The BWC builds always fetch the latest from the elastic/elasticsearch
repository for the BWC branches. Yet, there are use-cases for using the
local checkout without fetching the latest. This commit enables these
use-cases by adding a tests.bwc.git.fetch.latest property to skip the
fetches.

TESTING.asciidoc

@@ -498,6 +498,13 @@ will contain your change.
 . Push both branches to your remote repository.
 . Run the tests with `./gradlew check -Dtests.bwc.remote=${remote} -Dtests.bwc.refspec.5.x=index_req_bwc_5.x`.
 
+== Skip fetching latest
+
+For some BWC testing scenarios, you want to use the local clone of the
+repository without fetching latest. For these use cases, you can set the system
+property `tests.bwc.git_fetch_latest` to `false` and the BWC builds will skip
+fetching the latest from the remote.
+
 == Test coverage analysis
 
 Generating test coverage reports for Elasticsearch is currently not possible through Gradle.

distribution/bwc/build.gradle

@@ -54,6 +54,16 @@ subprojects {
 
   final String remote = System.getProperty("tests.bwc.remote", "elastic")
 
+  final boolean gitFetchLatest
+  final String gitFetchLatestProperty = System.getProperty("tests.bwc.git_fetch_latest", "true")
+  if ("true".equals(gitFetchLatestProperty)) {
+    gitFetchLatest = true
+  } else if ("false".equals(gitFetchLatestProperty)) {
+    gitFetchLatest = false
+  } else {
+    throw new GradleException("tests.bwc.git_fetch_latest must be [true] or [false] but was [" + gitFetchLatestProperty + "]")
+  }
+
   task createClone(type: LoggedExec) {
     onlyIf { checkoutDir.exists() == false }
     commandLine = ['git', 'clone', rootDir, checkoutDir]
@@ -83,7 +93,7 @@ subprojects {
   }
 
   task fetchLatest(type: LoggedExec) {
-    onlyIf { project.gradle.startParameter.isOffline() == false }
+    onlyIf { project.gradle.startParameter.isOffline() == false && gitFetchLatest }
     dependsOn addRemote
     workingDir = checkoutDir
     commandLine = ['git', 'fetch', '--all']