ApplyCommand: handle completely empty context lines in text patches

C git treats completely empty lines as empty context lines (which
traditionally have a single blank). Apparently newer GNU diff may
produce such lines; see [1]. ("Newer" meaning "since 2006"...)

[1] https://github.com/git/git/commit/b507b465f7831

Change-Id: I80c1f030edb17a46289b1dabf11a2648d2660d38
Signed-off-by: Thomas Wolf <thomas.wolf@paranor.ch>

org.eclipse.jgit.test/tst/org/eclipse/jgit/api/ApplyCommandTest.java

@@ -295,6 +295,14 @@ public void testEncodingChange() throws Exception {
 		checkBinary("umlaut", true);
 	}
 
+	@Test
+	public void testEmptyLine() throws Exception {
+		// C git accepts completely empty lines as empty context lines.
+		// According to comments in the C git sources (apply.c), newer GNU diff
+		// may produce such diffs.
+		checkBinary("emptyLine", true);
+	}
+
 	@Test
 	public void testAddA1() throws Exception {
 		ApplyResult result = init("A1", false, true);

org.eclipse.jgit/src/org/eclipse/jgit/api/ApplyCommand.java

@@ -640,6 +640,11 @@ && canApplyAt(hunkLines, newLines, 0)) {
 			int sz = hunkLines.size();
 			for (int j = 1; j < sz; j++) {
 				ByteBuffer hunkLine = hunkLines.get(j);
+				if (!hunkLine.hasRemaining()) {
+					// Completely empty line; accept as empty context line
+					applyAt++;
+					continue;
+				}
 				switch (hunkLine.array()[hunkLine.position()]) {
 				case ' ':
 					applyAt++;
@@ -676,8 +681,7 @@ && canApplyAt(hunkLines, newLines, 0)) {
 						// Must be the marker for the final newline
 						break;
 					}
-					out.write(line.array(), line.position(),
-							line.limit() - line.position());
+					out.write(line.array(), line.position(), line.remaining());
 					if (l.hasNext()) {
 						out.write('\n');
 					}
@@ -703,6 +707,14 @@ private boolean canApplyAt(List<ByteBuffer> hunkLines,
 		int pos = line;
 		for (int j = 1; j < sz; j++) {
 			ByteBuffer hunkLine = hunkLines.get(j);
+			if (!hunkLine.hasRemaining()) {
+				// Empty line. Accept as empty context line.
+				if (pos >= limit || newLines.get(pos).hasRemaining()) {
+					return false;
+				}
+				pos++;
+				continue;
+			}
 			switch (hunkLine.array()[hunkLine.position()]) {
 			case ' ':
 			case '-':