Fix PatienceDiffTest

Because PatienceDiff works by looking for common unique lines within
the region, the DiffTestDataGenerator needs to be modified to produce
a unique character for each region.  If we don't give PatienceDiff
a few unique points, it will just offer back a single REPLACE edit
that covers the entire files, and this doesn't tell us very much.

Change-Id: I5129faea1e763c74739118ca20d86bd62e0deaef
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>

org.eclipse.jgit.test/tst/org/eclipse/jgit/diff/DiffTestDataGenerator.java

@@ -78,8 +78,11 @@ public class DiffTestDataGenerator {
 		StringBuilder text = new StringBuilder(len);
 		int skipStart = skipPeriod - skipLength;
 		int skippedChars = 0;
+		int block = 0;
 		for (int i = 0; i - skippedChars < len; ++i) {
-			if (skipPeriod == 0 || i % skipPeriod < skipStart) {
+			if ((i % skipPeriod) == 1)
+				text.append((char) (256 + block++));
+			else if (skipPeriod == 0 || i % skipPeriod < skipStart) {
 				text.append((char) (32 + i % 95));
 			} else {
 				skippedChars++;