mformat: force multiline arguments with comma

Force multiline arguments when there is a trailing comma.
This is the behavior of muon.

Fixes #14721.

mesonbuild/mformat.py

@@ -242,6 +242,10 @@ class MultilineArgumentDetector(FullAstVisitor):
         if node.is_multiline:
             self.is_multiline = True
 
+        nargs = len(node)
+        if nargs and nargs == len(node.commas):
+            self.is_multiline = True
+
         if self.is_multiline:
             return
 

test cases/format/1 default/indentation.meson

@@ -92,3 +92,11 @@ if meson.project_version().version_compare('>1.2')
         # comment
     endif
 endif
+
+# Test for trailing comma:
+m = [1, 2, 3]
+n = [
+    1,
+    2,
+    3,
+]