Fix infinite loop in PatienceDiff

Certain inputs caused an infinite loop because the prior match data
couldn't be used as expected.  Rather than incrementing the match
pointer before looking at an element, do it after, so the loop breaks
when we wrap around to the starting point.

Change-Id: Ieab28bb3485a914eeddc68aa38c256f255dd778c
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit/src/org/eclipse/jgit/diff/PatienceDiffIndex.java

@@ -244,7 +244,7 @@ final class PatienceDiffIndex<S extends Sequence> {
 	private void scanA() {
 		int ptr = region.beginA;
 		final int end = region.endA;
-		int pLast = pBegin - 1;
+		int pLast = pBegin;
 		SCAN: while (ptr < end) {
 			final int tIdx = hash(a, ptr);
 
@@ -276,12 +276,7 @@ final class PatienceDiffIndex<S extends Sequence> {
 					// fact that pCommon is sorted by B, and its likely that
 					// matches in A appear in the same order as they do in B.
 					//
-					for (int pIdx = pLast + 1;; pIdx++) {
+					for (int pIdx = pLast;;) {
-						if (pIdx == pEnd)
-							pIdx = pBegin;
-						else if (pIdx == pLast)
-							break;
-
 						final long priorRec = pCommon[pIdx];
 						final int priorB = bOf(priorRec);
 						if (bs < priorB)
@@ -291,6 +286,12 @@ final class PatienceDiffIndex<S extends Sequence> {
 							pLast = pIdx;
 							continue SCAN;
 						}
+
+						pIdx++;
+						if (pIdx == pEnd)
+							pIdx = pBegin;
+						if (pIdx == pLast)
+							break;
 					}
 				}