feat: Add support for gerrit changes for download Jenkins file

src/main/java/hudson/plugins/git/BranchSpec.java

@@ -159,6 +159,9 @@ private Pattern getPattern(EnvVars env, String repositoryName) {
             String regexSubstring = expandedName.substring(1, expandedName.length());
             return Pattern.compile(regexSubstring);
         }
+        if (expandedName.startsWith("refs/changes/")) {
+            return Pattern.compile(Pattern.quote(expandedName));
+        }
         if (repositoryName != null) {
             // remove the "refs/.../" stuff from the branch-spec if necessary
             String pattern = cutRefs(expandedName)

src/main/java/jenkins/plugins/git/GitSCMFileSystem.java

@@ -267,6 +267,9 @@
                         || gscm.getBranches().get(0).getName().matches(
                             "^((\\Q" + Constants.R_TAGS + "\\E.*)|([^/]+)|(\\*/[^/*]+(/[^/*]+)*))$"
                         )
+                        || gscm.getBranches().get(0).getName().matches(
+                            "^((\\Qrefs/changes/\\E.*)|([^/]+)|(\\*/[^/*]+(/[^/*]+)*))$"
+                        )
                     );
             // we only support where the branch spec is obvious and not a wildcard
         }
@@ -306,6 +309,8 @@
                 String prefix = Constants.R_HEADS;
                 if (branchSpecExpandedName.startsWith(Constants.R_TAGS)) {
                     prefix = Constants.R_TAGS;
+                } else if (branchSpecExpandedName.startsWith("refs/changes")) {
+                    prefix = "refs/changes/";
                 }
 
                 String headName;

src/main/resources/hudson/plugins/git/BranchSpec/help-name.html

@@ -47,6 +47,10 @@
            Tracks/checks out the specified tag.<br/>
            E.g. <code>refs/tags/git-2.3.0</code>
       </li>
+      <li> <strong><code>refs/changes/&lt;gerritChange&gt;</code></strong><br/>
+           Tracks/checks out the specified gerrit change.<br/>
+           E.g. <code>refs/changes/91/45391/1</code>
+      </li>
       <li> <strong><code>&lt;commitId&gt;</code></strong><br/>
            Checks out the specified commit.<br/>
            E.g. <code>5062ac843f2b947733e6a3b105977056821bd352</code>, <code>5062ac84</code>, ...

src/test/java/hudson/plugins/git/BranchSpecTest.java

@@ -149,6 +149,14 @@ void testUsesRefsHeads() {
     	assertFalse(m.matches("remote/origin/jane"));
     }
 
+    @Test
+    public void testMatchesGerritChangeRef() {
+        BranchSpec spec = new BranchSpec("refs/changes/91/45391/1");
+
+        assertTrue(spec.matches("refs/changes/91/45391/1"));
+        assertFalse(spec.matches("refs/heads/refs/changes/91/45391/1"));
+    }
+
     @Test
     void testUsesJavaPatternDirectlyIfPrefixedWithColon() {
     	BranchSpec m = new BranchSpec(":^(?!(origin/prefix)).*");

src/test/java/jenkins/plugins/git/GitSCMFileSystemTest.java

@@ -435,6 +435,29 @@ void create_SCMFileSystem_from_tag() throws Exception {
         assertThat(file.contentAsString(), is("modified"));
     }
 
+    @Test
+    public void testSupportsGerritChangeRef() throws Exception {
+        // Create a dummy GitSCM configured with a Gerrit change refspec
+        GitSCM scm = new GitSCM(
+                GitSCM.createRepoList("https://example.com/repo.git", null),
+                Collections.singletonList(new BranchSpec("refs/changes/91/45391/1")),
+                null, null, Collections.emptyList());
+
+        GitSCMFileSystem.BuilderImpl builder = new GitSCMFileSystem.BuilderImpl();
+        assertTrue(builder.supports(scm), "Builder should support refs/changes/ branch specs");
+    }
+
+    @Test
+    public void testHeadNameResultCalculateGerritChange() {
+        BranchSpec spec = new BranchSpec("refs/changes/91/45391/1");
+
+        GitSCMFileSystem.BuilderImpl.HeadNameResult result =
+                GitSCMFileSystem.BuilderImpl.HeadNameResult.calculate(spec, null, null);
+
+        assertEquals("refs/changes/", result.prefix);
+        assertEquals("91/45391/1", result.headName);
+    }
+
     @Issue("JENKINS-52964")
     @Test
     void filesystem_supports_descriptor() throws Exception {