git-completion: Add git help completion for aliases

Enable bash completion for "git help <alias>", analogous to "git
<alias>", which was already implemented.

Signed-off-by: Jakob Pfender <jpfender@elegosoft.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -1508,7 +1508,7 @@ _git_help ()
 		;;
 	esac
 	__git_compute_all_commands
-	__gitcomp "$__git_all_commands
+	__gitcomp "$__git_all_commands $(__git_aliases)
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