Add sparseCheckout symbol for Pipeline (jenkinsci#1638)

Includes a test that confirms the sparseCheckout syntax works as expected.

Tested interactively to confirm that the new symbol behaves the same as
the old syntax behaved.  Does not include detailed functional testing
that the sparseCheckout symbol works as expected, since there are already
tests of sparse checkout in other tests.

README.adoc

@@ -159,7 +159,7 @@ This removes remote tracking branches and tags from the local workspace if they
 ----
 checkout scmGit(
     branches: [[name: 'master']],
-    extensions: [pruneStaleBranch(), pruneTags(true)],
+    extensions: [ pruneStaleBranch(), pruneTags(true) ],
     userRemoteConfigs: [[url: 'https://github.com/jenkinsci/ws-cleanup-plugin']])
 ----
 
@@ -1058,6 +1058,16 @@ Path::
 
   File or directory to be included in the checkout
 
+[source,groovy]
+----
+checkout scmGit(
+    branches: [[name: 'master']],
+    extensions: [
+        sparseCheckout(sparseCheckoutPaths: [[path: 'src'], [path: 'Makefile']])
+    ],
+    userRemoteConfigs: [[url: 'https://github.com/jenkinsci/git-plugin.git']])
+----
+
 [#git-lfs-pull-after-checkout]
 ==== Git LFS pull after checkout
 

src/main/java/hudson/plugins/git/extensions/impl/SparseCheckoutPaths.java

@@ -13,6 +13,7 @@
 import org.jenkinsci.plugins.gitclient.GitClient;
 import org.jenkinsci.plugins.gitclient.UnsupportedCommand;
 import org.jenkinsci.plugins.scriptsecurity.sandbox.whitelists.Whitelisted;
+import org.jenkinsci.Symbol;
 import org.kohsuke.stapler.DataBoundConstructor;
 import edu.umd.cs.findbugs.annotations.NonNull;
 
@@ -52,10 +53,11 @@ public void determineSupportForJGit(GitSCM scm, @NonNull UnsupportedCommand cmd)
     }
 
     @Extension
+    @Symbol("sparseCheckout")
     public static class DescriptorImpl extends GitSCMExtensionDescriptor {
         @Override
         public String getDisplayName() {
-            return "Sparse Checkout paths";
+            return "Sparse checkout paths";
         }
     }
 

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

@@ -163,6 +163,7 @@ private String randomPipelineExtensions() {
             "pruneTags()",
             "pruneTags(false)",
             "pruneTags(true)",
+            "sparseCheckout(sparseCheckoutPaths: [[path: 'src'], [path: 'Makefile']])",
             "submodule(disableSubmodules: true)",
             "submodule(depth: 1, shallow: true)",
             "submodule(parentCredentials: true, recursiveSubmodules: true, threads: 13)",