Fix transport version tests on windows (elastic#134739)

Running on windows has some slightly different behavior when interacting
with the filesystem. In particular, this commit fixes the changed
resources prefix when looking at changed resources in git to always use
forward slashes as git outputs them. It also fixes the case that CI uses
a non-main (eg master) branch as the default branch. Finally it fixes
git diff so it outputs paths relative to the transport resources dir, as
ls-files does.

Author: rjernst

build-tools-internal/src/integTest/groovy/org/elasticsearch/gradle/internal/transport/AbstractTransportVersionFuncTest.groovy

@@ -155,7 +155,11 @@ class AbstractTransportVersionFuncTest extends AbstractGradleFuncTest {
         """
 
         setupLocalGitRepo()
-        execute("git checkout -b main")
+        String currentBranch = execute("git branch --show-current")
+        if (currentBranch.strip().equals("main") == false) {
+            // make sure a main branch exists, some CI doesn't have main set as the default branch
+            execute("git checkout -b main")
+        }
         execute("git checkout -b test")
     }
 

build-tools-internal/src/main/java/org/elasticsearch/gradle/internal/transport/TransportVersionResourcesService.java

@@ -122,7 +122,8 @@ TransportVersionDefinition getReferableDefinitionFromUpstream(String name) {
     /** Get the definition names which have local changes relative to upstream */
     List<String> getChangedReferableDefinitionNames() {
         List<String> changedDefinitions = new ArrayList<>();
-        String referablePrefix = REFERABLE_DIR.toString();
+        // make sure the prefix is git style paths, always forward slashes
+        String referablePrefix = REFERABLE_DIR.toString().replace('\\', '/');
         for (String changedPath : getChangedResources()) {
             if (changedPath.contains(referablePrefix) == false) {
                 continue;
@@ -304,7 +305,7 @@ private Set<String> getChangedResources() {
             synchronized (changedResources) {
                 HashSet<String> resources = new HashSet<>();
 
-                String diffOutput = gitCommand("diff", "--name-only", getUpstreamRefName(), ".");
+                String diffOutput = gitCommand("diff", "--name-only", "--relative", getUpstreamRefName(), ".");
                 if (diffOutput.strip().isEmpty() == false) {
                     Collections.addAll(resources, diffOutput.split("\n")); // git always outputs LF
                 }