Support renames in grgit.show()

Fix #74, but using JGit's RenameDetector to process the commit's
diff before returning it to the caller.

src/main/groovy/org/ajoberstar/grgit/operation/ShowOp.groovy

@@ -25,6 +25,7 @@ import org.ajoberstar.grgit.util.JGitUtil
 
 import org.eclipse.jgit.diff.DiffEntry
 import org.eclipse.jgit.diff.DiffEntry.ChangeType
+import org.eclipse.jgit.diff.RenameDetector
 import org.eclipse.jgit.treewalk.TreeWalk
 
 /**
@@ -76,14 +77,19 @@ class ShowOp implements Callable<CommitDiff> {
 		if (parentId) {
 			walk.addTree(parentId.tree)
 			walk.addTree(commitId.tree)
-			Map entries = DiffEntry.scan(walk).groupBy { it.changeType }
+			List initialEntries = DiffEntry.scan(walk)
+			RenameDetector detector = new RenameDetector(repo.jgit.repository)
+			detector.addAll(initialEntries)
+			List entries = detector.compute()
+			Map entriesByType = entries.groupBy { it.changeType }
+
 			return new CommitDiff(
 				commit: commit,
-				added: entries[ChangeType.ADD].collect { it.newPath },
-				copied: entries[ChangeType.COPY].collect { it.newPath },
-				modified: entries[ChangeType.MODIFY].collect { it.newPath },
-				removed: entries[ChangeType.DELETE].collect { it.oldPath },
-				renamed: entries[ChangeType.RENAME].collect { it.newPath }
+				added: entriesByType[ChangeType.ADD].collect { it.newPath },
+				copied: entriesByType[ChangeType.COPY].collect { it.newPath },
+				modified: entriesByType[ChangeType.MODIFY].collect { it.newPath },
+				removed: entriesByType[ChangeType.DELETE].collect { it.oldPath },
+				renamed: entriesByType[ChangeType.RENAME].collect { it.newPath }
 			)
 		} else {
 			walk.addTree(commitId.tree)

src/test/groovy/org/ajoberstar/grgit/operation/ShowOpSpec.groovy

@@ -92,6 +92,24 @@ class ShowOpSpec extends SimpleGitOpSpec {
 		)
 	}
 
+	def 'can show diffs in commit with rename'() {
+		given:
+		repoFile('elephant.txt') << 'I have tusks.'
+		grgit.add(patterns: ['.'])
+		grgit.commit(message: 'Adding elephant.')
+
+		repoFile('elephant.txt').renameTo(repoFile('mammoth.txt'))
+		grgit.add(patterns: ['.'])
+		grgit.remove(patterns: ['elephant.txt'])
+		Commit renameCommit = grgit.commit(message: 'Renaming to mammoth.')
+
+		expect:
+		grgit.show(commit: renameCommit) == new CommitDiff(
+			commit: renameCommit,
+			renamed: ['mammoth.txt']
+		)
+	}
+
 	def 'can show diffs based on rev string'() {
 		File fooFile = repoFile("foo.txt")
 		fooFile << "foo!"