Improve name generation in the injection' tactic.

The tactic "injection' H" now uses the name "H" for the first hypothesis it
generates. Fresh names will still be used for the remaining hypotheses.

theories/tactics.v

@@ -115,7 +115,8 @@ Ltac unblock_hyps := unfold block in * |-.
 
 (** The tactic [injection' H] is a variant of injection that introduces the
 generated equalities. *)
-Ltac injection' H := block_goal; injection H; clear H; intros; unblock_goal.
+Ltac injection' H :=
+  block_goal; injection H; clear H; intros H; intros; unblock_goal.
 
 (** The tactic [simplify_equality] repeatedly substitutes, discriminates,
 and injects equalities, and tries to contradict impossible inequalities. *)