Git shallow clone for better performance

Do not download the entire commit history from the remote.  Execute the
equivalent of `git clone --depth 1`

Fixes spring-cloud#1544

docs/modules/ROOT/pages/server/environment-repository/git-backend.adoc

@@ -189,6 +189,20 @@ All other repositories are not cloned until configuration from the repository is
 NOTE: Setting a repository to be cloned when the Config Server starts up can help to identify a misconfigured configuration source (such as an invalid repository URI) quickly, while the Config Server is starting up.
 With `cloneOnStart` not enabled for a configuration source, the Config Server may start successfully with a misconfigured or invalid configuration source and not detect an error until an application requests configuration from that configuration source.
 
+By default, the server clones the entire commit history from remote repositories.
+Downloading a huge commit history might be slow, so the server can be configured to truncate the commit history in the clone to one commit, as shown in the following top-level example:
+
+[source,yaml]
+----
+spring:
+  cloud:
+    config:
+      server:
+        git:
+          uri: https://github.com/spring-cloud-samples/config-repo
+          clone-shallow: true
+----
+
 [[authentication]]
 == Authentication
 

spring-cloud-config-server/src/main/java/org/springframework/cloud/config/server/environment/JGitEnvironmentProperties.java

@@ -28,6 +28,7 @@
 /**
  * @author Dylan Roberts
  * @author Gareth Clay
+ * @author Chin Huang
  */
 public class JGitEnvironmentProperties extends AbstractScmAccessorProperties
 		implements HttpEnvironmentRepositoryProperties {
@@ -56,6 +57,13 @@ public class JGitEnvironmentProperties extends AbstractScmAccessorProperties
 	 */
 	private boolean cloneOnStart = false;
 
+	/**
+	 * Flag to indicate that the commit history in the clone should be truncated to one
+	 * commit. Generally leads to faster queries because the entire commit history is not
+	 * downloaded from the remote.
+	 */
+	private boolean cloneShallow = false;
+
 	/**
 	 * Flag to indicate that the submodules in the repository should be cloned.
 	 */
@@ -148,6 +156,14 @@ public void setCloneOnStart(boolean cloneOnStart) {
 		this.cloneOnStart = cloneOnStart;
 	}
 
+	public boolean isCloneShallow() {
+		return this.cloneShallow;
+	}
+
+	public void setCloneShallow(boolean cloneShallow) {
+		this.cloneShallow = cloneShallow;
+	}
+
 	public boolean isCloneSubmodules() {
 		return this.cloneSubmodules;
 	}

spring-cloud-config-server/src/main/java/org/springframework/cloud/config/server/environment/JGitEnvironmentRepository.java

@@ -77,6 +77,7 @@
  * @author Ryan Lynch
  * @author Gareth Clay
  * @author ChaoDong Xi
+ * @author Chin Huang
  */
 public class JGitEnvironmentRepository extends AbstractScmEnvironmentRepository
 		implements EnvironmentRepository, SearchPathLocator, InitializingBean {
@@ -159,7 +160,7 @@ public JGitEnvironmentRepository(ConfigurableEnvironment environment, JGitEnviro
 		this.deleteUntrackedBranches = properties.isDeleteUntrackedBranches();
 		this.refreshRate = properties.getRefreshRate();
 		this.skipSslValidation = properties.isSkipSslValidation();
-		this.gitFactory = new JGitFactory(properties.isCloneSubmodules());
+		this.gitFactory = new JGitFactory(properties.isCloneShallow(), properties.isCloneSubmodules());
 		this.tryMasterBranch = properties.isTryMasterBranch();
 		this.observationRegistry = observationRegistry;
 	}
@@ -756,13 +757,16 @@ public void setLastRefresh(long lastRefresh) {
 	 */
 	public static class JGitFactory {
 
+		private final boolean cloneShallow;
+
 		private final boolean cloneSubmodules;
 
 		public JGitFactory() {
-			this(false);
+			this(false, false);
 		}
 
-		public JGitFactory(boolean cloneSubmodules) {
+		public JGitFactory(boolean cloneShallow, boolean cloneSubmodules) {
+			this.cloneShallow = cloneShallow;
 			this.cloneSubmodules = cloneSubmodules;
 		}
 
@@ -773,6 +777,9 @@ public Git getGitByOpen(File file) throws IOException {
 
 		public CloneCommand getCloneCommandByCloneRepository() {
 			CloneCommand command = Git.cloneRepository().setCloneSubmodules(cloneSubmodules);
+			if (cloneShallow) {
+				command.setDepth(1);
+			}
 			return command;
 		}