feat: Add support for gerrit changes for download Jenkins file

We want to support the verification pipeline when the Jenkinsfile
is part of the modification itself. This allow to test the
pipeline on any project and test it.

Signed-off-by: Michael Trimarchi <michael@amarulasolutions.com>

Author: panicking

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 @@ public boolean supports(SCM source) {
                         || 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 @@ static HeadNameResult calculate(@NonNull BranchSpec branchSpec,
                 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>, ...