Add configuration parameter to enable built-in hooks/filters

If the configuration parameter filter.<filterDriverName>.useJGitBuiltin
is set to true then for all corresponding filters JGit will try to
execute the built-in filter instead of the filter-command which is
defined in git configuration. It will fallback to the non-built-in
filters if no built-in filters are registered or if constructing them
leads to exceptions. If set to false JGit will not try to execute
built-in filters for the specified filter driver.

Example: The configuration contains the following lines

  [filter "lfs"]
    clean = git-lfs clean -- %f
    smudge = git-lfs smudge -- %f
    useJGitBuiltin = true

Addtionally the .gitattributes file in the root of the working tree
contains:

    *.bin filter=lfs

In this case when new content is added similar to "git add 1.bin" then
the following will happen:

  - jgit will check whether a built-in command factory was registered
    for the command "jgit://builtin/lfs/clean". If that is true the
    factory is used to create a built-in filter command and that
    command is used to filter the content
  - Otherwise jgit will call the external program "git lfs clean ..."
    to do the filtering

Change-Id: Idadb1db06b1e89e7031d7ed6319904973c367d38

org.eclipse.jgit/src/org/eclipse/jgit/attributes/FilterCommandRegistry.java

@@ -45,6 +45,7 @@ package org.eclipse.jgit.attributes;
 import java.io.IOException;
 import java.io.InputStream;
 import java.io.OutputStream;
+import java.util.Set;
 import java.util.concurrent.ConcurrentHashMap;
 
 import org.eclipse.jgit.lib.Repository;
@@ -102,6 +103,14 @@ public class FilterCommandRegistry {
 		return filterCommandRegistry.containsKey(filterCommandName);
 	}
 
+	/**
+	 * @return Set of commandNames for which a {@link FilterCommandFactory} is
+	 *         registered
+	 */
+	public static Set<String> getRegisteredFilterCommands() {
+		return filterCommandRegistry.keySet();
+	}
+
 	/**
 	 * Creates a new {@link FilterCommand} for the given name. A factory must be
 	 * registered for the name in advance.

org.eclipse.jgit/src/org/eclipse/jgit/lib/Constants.java

@@ -391,6 +391,13 @@ public final class Constants {
 	 */
 	public static final String ATTR_FILTER_TYPE_SMUDGE = "smudge";
 
+	/**
+	 * Whether to use JGit's implementations of filters and hooks
+	 *
+	 * @since 4.6
+	 */
+	public static final String ATTR_FILTER_USE_BUILTIN = "useJGitBuiltin";
+
 	/**
 	 * Builtin filter commands start with this prefix
 	 *

org.eclipse.jgit/src/org/eclipse/jgit/treewalk/TreeWalk.java

@@ -55,6 +55,7 @@ import org.eclipse.jgit.attributes.Attribute;
 import org.eclipse.jgit.attributes.Attributes;
 import org.eclipse.jgit.attributes.AttributesNodeProvider;
 import org.eclipse.jgit.attributes.AttributesProvider;
+import org.eclipse.jgit.attributes.FilterCommandRegistry;
 import org.eclipse.jgit.dircache.DirCacheBuildIterator;
 import org.eclipse.jgit.attributes.AttributesHandler;
 import org.eclipse.jgit.dircache.DirCacheIterator;
@@ -313,6 +314,8 @@ public class TreeWalk implements AutoCloseable, AttributesProvider {
 
 	private Config config;
 
+	private Set<String> filterCommands;
+
 	/**
 	 * Create a new tree walker for a given repository.
 	 *
@@ -357,6 +360,8 @@ public class TreeWalk implements AutoCloseable, AttributesProvider {
 		if (repo != null) {
 			config = repo.getConfig();
 			attributesNodeProvider = repo.createAttributesNodeProvider();
+			filterCommands = FilterCommandRegistry
+					.getRegisteredFilterCommands();
 		} else {
 			config = null;
 			attributesNodeProvider = null;
@@ -1369,8 +1374,19 @@ public class TreeWalk implements AutoCloseable, AttributesProvider {
 			return filterCommand;
 		filterCommand = config.getString(Constants.ATTR_FILTER,
 				filterDriverName, filterCommandType);
-		if (filterCommand != null)
+		boolean useBuiltin = config.getBoolean(Constants.ATTR_FILTER,
+				filterDriverName, Constants.ATTR_FILTER_USE_BUILTIN, false);
+		if (useBuiltin) {
+			String builtinFilterCommand = Constants.BUILTIN_FILTER_PREFIX
+					+ filterDriverName + '/' + filterCommandType;
+			if (filterCommands != null
+					&& filterCommands.contains(builtinFilterCommand)) {
+				filterCommand = builtinFilterCommand;
+			}
+		}
+		if (filterCommand != null) {
 			filterCommandsByNameDotType.put(key, filterCommand);
+		}
 		return filterCommand;
 	}
 }