Add option to select diff algorithm for diff command

The diff command in the pgm package was enhanced to allow
choosing the diff algorithm (currently myers or histogram)

Change-Id: I72083e78fb5c92868eb5d8ec512277d212a39349
Signed-off-by: Christian Halstrick <christian.halstrick@sap.com>

org.eclipse.jgit.pgm/resources/org/eclipse/jgit/pgm/CLIText.properties

 usage_deleteBranchEvenIfNotMerged=delete branch (even if not merged)
 usage_deleteFullyMergedBranch=delete fully merged branch
 usage_detectRenames=detect renamed files
+usage_diffAlgorithm=the diff algorithm to use
 usage_directoriesToExport=directories to export
 usage_disableTheServiceInAllRepositories=disable the service in all repositories
 usage_displayAListOfAllRegisteredJgitCommands=Display a list of all registered jgit commands

org.eclipse.jgit.pgm/src/org/eclipse/jgit/pgm/Diff.java

 import java.text.MessageFormat;
 import java.util.List;
 
+import org.eclipse.jgit.diff.DiffAlgorithm;
 import org.eclipse.jgit.diff.DiffEntry;
 import org.eclipse.jgit.diff.DiffFormatter;
+import org.eclipse.jgit.diff.HistogramDiff;
+import org.eclipse.jgit.diff.MyersDiff;
 import org.eclipse.jgit.diff.RawTextComparator;
 import org.eclipse.jgit.diff.RenameDetector;
 import org.eclipse.jgit.dircache.DirCacheIterator;
 		detectRenames = Boolean.FALSE;
 	}
 
+	enum SupportedAlgorithm {
+		myers(MyersDiff.INSTANCE), histogram(new HistogramDiff());
+
+		public DiffAlgorithm algorithm;
+
+		SupportedAlgorithm(DiffAlgorithm a) {
+			algorithm = a;
+		}
+	};
+
+	@Option(name = "--algorithm", usage = "usage_diffAlgorithm")
+	void setAlgorithm(SupportedAlgorithm s) {
+		diffFmt.setDiffAlgorithm(s.algorithm);
+	}
+
 	@Option(name = "-l", usage = "usage_renameLimit")
 	private Integer renameLimit;