use Set instead of List to eliminate duplicates

One use-case is just selecting all requests in Proxy and invoking this
extension from the context menu. However, if several rows had the same
response content, they'd get checked more than once -- even though it
doesn't help the end result in any way. Using a Set removes duplicates
since ObjectId implements a sane .equals() method. A HashSet performs
this rather well, since ObjectId uses the second 32-bit word from the ID
it represents, resulting in a rather uniform distribution.

src/burp/BurpExtender.java

@@ -78,7 +78,7 @@ public Session(Frame owner, File gitDir, IHttpRequestResponse[] messages) {
 		public void run() {
 			try {
 				long start = System.currentTimeMillis();
-				List<ObjectId> conditions = messagesToConditions(messages);
+				Iterable<ObjectId> conditions = messagesToConditions(messages);
 				Set<RevCommit> cs = findCommits(gitDir, conditions, this);
 				long delta = System.currentTimeMillis() - start;
 				ByteArrayOutputStream baos = new ByteArrayOutputStream();
@@ -160,8 +160,8 @@ private File pickGitDir(Frame owner) {
 		return gitDir;
 	}
 
-	private List<ObjectId> messagesToConditions(IHttpRequestResponse[] messages) throws NoSuchAlgorithmException {
-		List<ObjectId> conditions = new ArrayList<>(messages.length);
+	private Iterable<ObjectId> messagesToConditions(IHttpRequestResponse[] messages) throws NoSuchAlgorithmException {
+		Set<ObjectId> conditions = new HashSet<>(messages.length);
 		MessageDigest md = MessageDigest.getInstance("SHA");
 		for (IHttpRequestResponse messageInfo : messages) {
 			IHttpService hs = messageInfo.getHttpService();