BlameRegionMerger: report invalid regions with checked exception.

If the cached regions are invalid the merger throws an
IllegalStateException. This is too strict. The caller can just
continue working as if there was no cache.

Report the error as IOException, that the caller can catch and handle.

Change-Id: I19a1061225533b46d3a17936912a11000430f2ce

Author: ifradeo

org.eclipse.jgit.test/tst/org/eclipse/jgit/blame/BlameRegionMergerTest.java

@@ -174,7 +174,7 @@ public void blame_exactOverlap() throws IOException {
 	public void blame_corruptedIndex() {
 		Region outOfRange = new Region(0, 43, 4);
 		// This region is out of the blamed area
-		assertThrows(IllegalStateException.class,
+		assertThrows(IOException.class,
 				() -> blamer.mergeOneRegion(outOfRange));
 	}
 

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

@@ -72,7 +72,7 @@ Candidate mergeCandidate(Candidate candidate) {
 	List<Candidate> mergeOneRegion(Region region) throws IOException {
 		List<CacheRegion> overlaps = findOverlaps(region);
 		if (overlaps.isEmpty()) {
-			throw new IllegalStateException(
+			throw new IOException(
 					"Cached blame should cover all lines");
 		}
 		/*