checkout: take mode into account when comparing index to baseline

When checking out a file, we determine whether the baseline (what we
expect to be in the working directory) actually matches the contents
of the working directory.  This is safe behavior to prevent us from
overwriting changes in the working directory.

We look at the index to optimize this test: if we know that the index
matches the working directory, then we can simply look at the index
data compared to the baseline.

We have historically compared the baseline to the index entry by oid.
However, we must also compare the mode of the two items to ensure that
they are identical.  Otherwise, we will refuse to update the working
directory for a mode change.

src/checkout.c

@@ -205,17 +205,23 @@ static bool checkout_is_workdir_modified(
 		return rval;
 	}
 
-	/* Look at the cache to decide if the workdir is modified.  If not,
-	 * we can simply compare the oid in the cache to the baseitem instead
-	 * of hashing the file.  If so, we allow the checkout to proceed if the
-	 * oid is identical (ie, the staged item is what we're trying to check
-	 * out.)
+	/*
+	 * Look at the cache to decide if the workdir is modified: if the
+	 * cache contents match the workdir contents, then we do not need
+	 * to examine the working directory directly, instead we can
+	 * examine the cache to see if _it_ has been modified.  This allows
+	 * us to avoid touching the disk.
 	 */
-	if ((ie = git_index_get_bypath(data->index, wditem->path, 0)) != NULL) {
-		if (git_index_time_eq(&wditem->mtime, &ie->mtime) &&
-			wditem->file_size == ie->file_size &&
-			!is_file_mode_changed(wditem->mode, ie->mode))
-			return !is_workdir_base_or_new(&ie->id, baseitem, newitem);
+	ie = git_index_get_bypath(data->index, wditem->path, 0);
+
+	if (ie != NULL &&
+		git_index_time_eq(&wditem->mtime, &ie->mtime) &&
+		wditem->file_size == ie->file_size &&
+		!is_file_mode_changed(wditem->mode, ie->mode)) {
+
+		/* The workdir is modified iff the index entry is modified */
+		return !is_workdir_base_or_new(&ie->id, baseitem, newitem) ||
+			is_file_mode_changed(baseitem->mode, ie->mode);
 	}
 
 	/* depending on where base is coming from, we may or may not know