Fix gitcommit highlighting for new parser

The "overflow" node type was removed in this PR:
gbprod/tree-sitter-gitcommit#68

nvim/after/queries/gitcommit/highlights.scm

@@ -1,4 +1,11 @@
 ; extends
 
-(subject (overflow) @gitcommit_error)
 (ERROR) @gitcommit_error
+
+((subject) @gitcommit_error
+  (#vim-match? @gitcommit_error ".\{50,}")
+  (#offset! @gitcommit_error 0 50 0 0))
+
+((message_line) @gitcommit_error
+  (#vim-match? @gitcommit_error ".\{72,}")
+  (#offset! @gitcommit_error 0 72 0 0))