grep open pull requests

I have a couple questions around grepping among open pull requests.

First, "git for-each-ref --no-merged": When I run the following, it lists refs/pull/1112/head, even though #1112 was merged in commit ced4da1. I guess this is because the tip of refs/pull/1112/head is 107fc59, not ced4da1?

This maybe shows my lack of familiarity with Git details, but superficially the two commits appear identical -- [1] and [2] -- same parent, etc. Nonetheless they have different SHA-1s. I'm not sure why that is -- I didn't merge the commit -- but regardless, GitHub somehow still connects ced4da1 with #1112.

So my question is, how are they doing that, and why can't "git for-each-ref --no-merged" likewise connect ced4da1 with refs/pull/1112/head?

   $ git clone \
   >   --bare \
   >   --config remote.origin.fetch=+refs/pull/*/head:refs/pull/*/head \
   >   https://github.com/apache/trafficserver.git
   $ cd trafficserver.git
   $ git for-each-ref --no-merged

More generally, what I want is to periodically list open pull requests that add or modify lines containing the string "memset". So far I have the following in a Makefile. Can you recommend any improvements?

.PHONY: all
all: trafficserver.git
	cd trafficserver.git && git fetch
	cd trafficserver.git && git for-each-ref --format '%(refname)' 
refs/pull --no-merged | \
	  while read refname; do \
	    git log -p "master..$$refname" | grep -q '^+.*memset' && echo 
"$$refname"; \
	  done

trafficserver.git:
	git clone \
	  --bare \
	  --config remote.origin.fetch=+refs/pull/*/head:refs/pull/*/head \
	  https://github.com/apache/trafficserver.git

Lastly, a question more about GitHub than Git, but: Given the way GitHub is setup, I hope I can get a list of unmerged pull requests from Git alone. Can you think of a way to list *open* pull requests, or is that status only available out of band?

Thanks!

[1] https://github.com/apache/trafficserver/commit/107fc59104cce2a4b527f04e7ac86695c98b568c [2] https://github.com/apache/trafficserver/commit/ced4da13279f834c381925f2ecd1649bfb459e8b