PCRE2 allows longer group names than PCRE1 did.
A failure case in .mtn-ignore became a valid lone ignoring everything.
Make it invalid again

--- mtn-src/test/func/syntax_errors_in_.mtn-ignore/mtn-ignore	2026-08-06 03:22:51.680338185 +0200
+++ mtn-src/test/func/syntax_errors_in_.mtn-ignore/mtn-ignore	2026-08-06 03:22:55.096806215 +0200
@@ -27,6 +27,6 @@
 (?R)
 (?P*)
 (?P<*>x)
-(?P<aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>)
+(?P<aaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaaa>)
 \777
 ^ignoremetoo$