add commented-out diff and rebase injection queries

runtime/queries/gitcommit/injections.scm

@@ -0,0 +1,15 @@
+; once a diff grammar is available, we can inject diff highlighting into the
+; trailer after scissors (git commit --verbose)
+; see https://github.com/helix-editor/helix/pull/1338#issuecomment-1000013539
+;
+; ((comment (scissors))
+;  (message) @injection.content
+;  (#set! injection.language "diff"))
+
+; ---
+
+; once a rebase grammar is available, we can inject rebase highlighting into
+; interactive rebase summary sections like so:
+;
+; ((rebase_command) @injection.content
+;  (#set! injection.language "git-rebase"))