mrhlpr.json: stop making the directory prefix compulsory (MR 5840)

This has not been a requirement since the introduction of COMMITSTYLE
guidelines in f2993c3a, so let's make
these kind of commits pass!

.mrhlpr.json

 {
     "subject_format": {
         "pass": [
-            "^(([a-z]+\\/[a-z\\-0-9*{}]+),?)+: [A-Za-z\\/\\-0-9*{}\\.,_&:=' ]+( \\(MR \\d+\\)|)$",
+            "^(([a-z]+\\/\\)?\\([a-z\\-0-9*{}]+),?)+: [A-Za-z\\/\\-0-9*{}\\.,_&:=' ]+( \\(MR \\d+\\)|)$",
             "^CI: [a-z-0-9*{}\\(\\)\\.,_ ][A-Za-z\\/\\-0-9*{}\\.,_ ]+( \\(MR \\d+\\)|)$",
             "^mrhlpr.json: [a-z-0-9*{}\\(\\)\\.,_ ][A-Za-z\\/\\-0-9*{}\\.,_ ]+( \\(MR \\d+\\)|)$",
             "^pmaports.cfg: [a-z-0-9*{}\\(\\)\\.,_ ][A-Za-z\\/\\-0-9*{}\\.,_ ]+( \\(MR \\d+\\)|)$"