checkout: break case-changes into delete/add

When checking out with a case-insensitive working directory, we
want to change the case of items in the working directory to
reflect changes that occured in the checkout target.  Diff now
has an option to break case-changing renames into delete/add.

include/git2/diff.h

@@ -124,6 +124,11 @@ typedef enum {
 	/** Use case insensitive filename comparisons */
 	GIT_DIFF_IGNORE_CASE = (1u << 10),
 
+	/** May be combined with `GIT_DIFF_IGNORE_CASE` to specify that a file
+	 *  that has changed case will be returned as an add/delete pair.
+	 */
+	GIT_DIFF_INCLUDE_CASECHANGE = (1u << 11),
+
 	/** If the pathspec is set in the diff options, this flags means to
 	 *  apply it as an exact match instead of as an fnmatch pattern.
 	 */

src/checkout.c

@@ -2485,7 +2485,8 @@ int git_checkout_iterator(
 		GIT_DIFF_INCLUDE_IGNORED |
 		GIT_DIFF_INCLUDE_TYPECHANGE |
 		GIT_DIFF_INCLUDE_TYPECHANGE_TREES |
-		GIT_DIFF_SKIP_BINARY_CHECK;
+		GIT_DIFF_SKIP_BINARY_CHECK |
+		GIT_DIFF_INCLUDE_CASECHANGE;
 	if (data.opts.checkout_strategy & GIT_CHECKOUT_DISABLE_PATHSPEC_MATCH)
 		diff_opts.flags |= GIT_DIFF_DISABLE_PATHSPEC_MATCH;
 	if (data.opts.paths.count > 0)

src/diff.c

@@ -822,6 +822,19 @@ static int maybe_modified(
 			status = GIT_DELTA_UNMODIFIED;
 	}
 
+	/* If we want case changes, then break this into a delete of the old
+	 * and an add of the new so that consumers can act accordingly (eg,
+	 * checkout will update the case on disk.)
+	 */
+	if (DIFF_FLAG_IS_SET(diff, GIT_DIFF_IGNORE_CASE) &&
+		DIFF_FLAG_IS_SET(diff, GIT_DIFF_INCLUDE_CASECHANGE) &&
+		strcmp(oitem->path, nitem->path) != 0) {
+
+		if (!(error = diff_delta__from_one(diff, GIT_DELTA_DELETED, oitem)))
+			error = diff_delta__from_one(diff, GIT_DELTA_ADDED, nitem);
+		return error;
+	}
+
 	return diff_delta__from_two(
 		diff, status, oitem, omode, nitem, nmode,
 		git_oid_iszero(&noid) ? NULL : &noid, matched_pathspec);