merge_causes_out_of_memory_on_large_repos.mdwn

### Please describe the problem.

`git annex merge` goes out of memory on a large repo.


### What steps will reproduce the problem?

    $ git annex merge
    merge git-annex git-annex: out of memory (requested 1048576 bytes)

### What version of git-annex are you using? On what operating system?

    git-annex version: 4.20130827
    build flags: Assistant Webapp Pairing Testsuite S3 WebDAV Inotify DBus XMPP DNS

On debian wheezy i386

### Please provide any additional information below.

The repository contains a lot of files:

    $ cd .git/objects
    $ find . -type f | wc -l 
    1091548