git -- how to revert build to as-originally-cloned?

I am stuck trying to revert a private kernel build back to the state in which I originally cloned it, (after probably doing the wrong thing - as below). Hoping someone can advise.

Here's what I did   (helpful criticism welcome)

On machine MA in filesystem /a  on 13 May

git clone git://git.kernel.org/pub/scm/linux/kernel/git/davem/net-next-2.6.git

(This build built ok and ran ok  and is what I want back)

A few days later on machine MB on filesystem /b - same git clone command but of course a slightly changed build.

Yesterday  -  I wanted to synch build a/ from b/  :
    git branch jel_r8169  /*  made a new branch --  ok */
    nfs-mounted /b on MA
    git fetch file:///a/.../net-next-2.6/.git   /*  worked  ok  *
    git merge FETCH_HEAD                        /*  worked ok and output 
list of files : */
               output started with
Updating 72a8f97..1b1cb1f
Fast-forward
               then list of files
               output ended with
   56 files changed, 3352 insertions(+), 886 deletions(-)
  create mode 100644 include/net/ping.h
  create mode 100644 net/ipv4/ping.c

I then built this build and built ok but build is broken - unresolved syms in some modules - I want to undo my merge.

I have tried all the commands I can find that claim to do this
and none of them have done it, e.g. :
   git reset --hard HEAD  /*  did nothing */
   git reset --hard ORIG_HEAD  /*  did nothing */

Not only that, but none of the various show , log , status commands appear to be aware of the merge at all. There appears to be no record of it - but the actual files themselves are the updated ones. (diff with /b compares equal)

How can I undo it?

Cheers    John Lumby