@@ -737,13 +737,9 @@ - (NSString *)readDiffForFile:(PBChangedFile *)file staged:(BOOL)staged contextL
 {
 	NSString *parameter = [NSString stringWithFormat:@"-U%lu", context];
 	if (staged) {
-		NSArray *arguments = nil;
-		if (file.status == NEW) {
-			NSString *indexPath = [@":0:" stringByAppendingString:file.path];
-			arguments = @[ @"show", indexPath ];
-		} else {
-			arguments = @[ @"diff-index", parameter, @"--cached", self.parentTree, @"--", file.path ];
-		}
+		// A staged addition is diffed against the parent tree (the empty tree in
+		// an unborn repository), not dumped from the index blob
+		NSArray *arguments = @[ @"diff-index", parameter, @"--cached", self.parentTree, @"--", file.path ];
 
 		NSError *error = nil;
 		NSString *output = [self.repository outputOfTaskWithArguments:arguments error:&error];
@@ -754,7 +750,10 @@ - (NSString *)readDiffForFile:(PBChangedFile *)file staged:(BOOL)staged contextL
 	}
 
 	// unstaged
-	if (file.status == NEW) {
+	// NEW only means absent from the parent tree; once staged there is an index
+	// entry to diff against, so only an untracked file is shown whole
+	BOOL untracked = file.status == NEW && !file.hasStagedChanges;
+	if (untracked) {
 		NSStringEncoding encoding;
 		NSError *error = nil;
 		NSURL *fileURL = [self.repository.workingDirectoryURL URLByAppendingPathComponent:file.path];
 
@@ -184,4 +184,41 @@ - (void)testAmendingIsAskedForAgain
 	XCTAssertEqual(self.repository.askedFor.count, 2u);
 }
 
+#pragma mark A staged addition is diffed, only an untracked file is read whole
+
+- (PBChangedFile *)newFileStaged:(BOOL)staged
+{
+	PBChangedFile *file = [[PBChangedFile alloc] initWithPath:@"a.txt"];
+	file.status = NEW;
+	file.hasStagedChanges = staged;
+	file.hasUnstagedChanges = YES;
+	return file;
+}
+
+- (void)testUntrackedFileIsReadFromDiskWithoutAskingGit
+{
+	NSString *diff = [self.gitIndex diffForFile:[self newFileStaged:NO] staged:NO contextLines:3];
+
+	XCTAssertEqual(self.repository.askedFor.count, 0u, @"git has no index entry to diff an untracked file against");
+	XCTAssertEqualObjects(diff, @"one\n");
+}
+
+- (void)testPartiallyStagedAdditionAsksForTheWorkingTreeDiff
+{
+	[self.gitIndex diffForFile:[self newFileStaged:YES] staged:NO contextLines:3];
+
+	NSArray *expected = @[ @"diff-files", @"-U3", @"--", @"a.txt" ];
+	XCTAssertEqualObjects(self.repository.askedFor.lastObject, expected,
+						  @"the edits since staging are a diff against the index, not the whole file again");
+}
+
+- (void)testStagedAdditionIsDiffedAgainstTheParentTree
+{
+	[self.gitIndex diffForFile:[self newFileStaged:YES] staged:YES contextLines:3];
+
+	NSArray *expected = @[ @"diff-index", @"-U3", @"--cached", @"HEAD", @"--", @"a.txt" ];
+	XCTAssertEqualObjects(self.repository.askedFor.lastObject, expected,
+						  @"shown as added lines like any staged change, not as a bare blob");
+}
+
 @end
@@ -61,7 +61,8 @@ var showFileChanges = function(file, cached) {
 		Controller.refresh();
 	}
 
-	if (file.status == 0) // New file?
+	// Only an untracked file is shown whole; a staged addition has a real diff on both sides
+	if (file.status == 0 && !file.hasStagedChanges) // Untracked file?
 		return showNewFile(file);
 
 	setTitle((cached ? "Staged" : "Unstaged") + " changes for " + file.path);
@@ -378,12 +379,32 @@ var stageLines = function(reverse) {
 			patch += l+"\n";
 		}
 	}
-	patch = diffHeader + '\n' + "@@ -" + start_old.toString() + "," + count[0].toString() +
+	var header = diffHeader;
+	if (start_old == 0 && count[0] > 0) {
+		// Old side empty (addition, empty file gaining lines, insert at top with
+		// context 0): git won't partially reverse a creation header, so unstage the
+		// selection as a modification of the index entry. A full selection keeps
+		// the header and removes the entry, like the hunk button
+		header = modificationHeader(diffHeader);
+		start_old = 1;
+	}
+	patch = header + '\n' + "@@ -" + start_old.toString() + "," + count[0].toString() +
 		" +" + start_new.toString() + "," + count[1].toString() + " @@\n"+patch;
 
 	addHunkText(patch,reverse);
 }
 
+/* Plain modification header for the path of any diff header. Git writes both
+ * names on the "diff --git A B" line the same length, so splitting at the
+ * middle is safe for paths with spaces or quotes */
+var modificationHeader = function(header)
+{
+	var names = header.split("\n")[0].replace(/^diff --git /, "");
+	var half = (names.length - 1) / 2;
+	var a = names.substr(0, half), b = names.substr(half + 1);
+	return "diff --git " + a + " " + b + "\n--- " + a + "\n+++ " + b;
+}
+
 /* Compute the selection before actually making it.  Return as object
  * with 2-element array "bounds", and "good", which indicates if the
  * selection contains add/del lines. */