include-children in diff injections

This ensures that children nodes of $.message are included in the
injection, such as $.user or issue/pr numbers. Without this change,
diffs containing '#' or '@' characters can trip up the injection and
be parsed separately.

See #1373 (comment)

runtime/queries/git-commit/injections.scm

@@ -1,5 +1,6 @@
 ((comment (scissors))
  (message) @injection.content
+ (#set! injection.include-children)
  (#set! injection.language "diff"))
 
 ; once a rebase grammar is available, we can inject rebase highlighting into