Consider task up-to-date on equal timestamps

Fixes issue #48 by considering a task up-to-date if the newest
file from sources and the oldest file from generates has
exactly the same time-stamp.

task.go

@@ -250,8 +250,7 @@ func (e *Executor) isUpToDateTimestamp(ctx context.Context, call Call) (bool, er
 	if err != nil || generatesMinTime.IsZero() {
 		return false, nil
 	}
-
-	return generatesMinTime.After(sourcesMaxTime), nil
+	return !generatesMinTime.Before(sourcesMaxTime), nil
 }
 
 func (e *Executor) runCommand(ctx context.Context, call Call, i int) error {

testdata/generates/Taskfile.yml

@@ -27,8 +27,5 @@ sub/src.txt:
   cmds:
     - mkdir -p sub
     - echo "hello world" > sub/src.txt
-    # Sleep to avoid time-stamp issue. Some filesystems, E.g. HFS+ for Mac OS,
-    # only support one second precision on file timestamps.
-    - sleep 1
   status:
     - test -f sub/src.txt