checkout: allow workdir to contain checkout target

When checking out some file 'foo' that has been modified in the
working directory, allow the checkout to proceed (do not conflict)
if 'foo' is identical to the target of the checkout.

src/checkout.c

@@ -150,6 +150,15 @@ static int checkout_notify(
 	}
 }
 
+GIT_INLINE(bool) is_workdir_base_or_new(
+	const git_oid *workdir_id,
+	const git_diff_file *baseitem,
+	const git_diff_file *newitem)
+{
+	return (git_oid__cmp(&baseitem->id, workdir_id) == 0 ||
+		git_oid__cmp(&newitem->id, workdir_id) == 0);
+}
+
 static bool checkout_is_workdir_modified(
 	checkout_data *data,
 	const git_diff_file *baseitem,
@@ -193,8 +202,7 @@ static bool checkout_is_workdir_modified(
 		if (wditem->mtime.seconds == ie->mtime.seconds &&
 			wditem->mtime.nanoseconds == ie->mtime.nanoseconds &&
 			wditem->file_size == ie->file_size)
-			return (git_oid__cmp(&baseitem->id, &ie->id) != 0 &&
-				git_oid_cmp(&newitem->id, &ie->id) != 0);
+			return !is_workdir_base_or_new(&ie->id, baseitem, newitem);
 	}
 
 	/* depending on where base is coming from, we may or may not know
@@ -206,7 +214,10 @@ static bool checkout_is_workdir_modified(
 	if (git_diff__oid_for_entry(&oid, data->diff, wditem, NULL) < 0)
 		return false;
 
-	return (git_oid__cmp(&baseitem->id, &oid) != 0);
+	/* Allow the checkout if the workdir is not modified *or* if the checkout
+	 * target's contents are already in the working directory.
+	 */
+	return !is_workdir_base_or_new(&oid, baseitem, newitem);
 }
 
 #define CHECKOUT_ACTION_IF(FLAG,YES,NO) \