blame: Only use computeRange if -L was requested

The computeRange method is inefficient for computing the entire file.
If the entire file was selected ask for the entire file.

Change-Id: I8b2dbf635e875cc125443dac50be121208646540

org.eclipse.jgit/src/org/eclipse/jgit/blame/BlameResult.java

@@ -290,6 +290,10 @@ public class BlameResult {
 		BlameGenerator gen = generator;
 		if (gen == null)
 			return;
+		if (start == 0 && end == resultContents.size()) {
+			computeAll();
+			return;
+		}
 
 		while (start < end) {
 			if (hasSourceData(start, end))