Repository tuning parameters can now be passed when initializing a repository for the first time.

* init: Repository tuning parameters can now be passed when initializing a
  repository for the first time. For details, see
  http://git-annex.branchable.com/tuning/
* merge: Refuse to merge changes from a git-annex branch of a repo
  that has been tuned in incompatable ways.

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+git-annex now has experimental support for tuning a repository for
+different work loads. 
+
+For example, a repository with a very large number of files in it may work
+better if git-annex uses some nonstandard hash format, for either the
+`.git/annex/objects/` directory, or for the log files in the git-annex
+branch.
+
+A repository can currently only be tuned when it is first created; this is
+done by passing `-c name=value` parameters to `git annex init`.
+
+For example, this will make git-annex use only 1 level for hash directories
+in `.git/annex/objects`:
+
+	git -c annex.tune.objecthashdirectories=1 annex init
+
+It's very important to keep in mind that this makes a nonstandard format
+git-annex repository. In general, this cannot safely be used with
+git-annex older than version 5.20150128. Older version of git-annex will
+not understand and will get confused and perhaps do bad things.
+
+Also, it's not safe to merge two separate git repositories that have been
+tuned differently (or one tuned and the other one not). git-annex will
+prevent merging their git-annex branches together, but it cannot prevent
+`git merge remote/master` merging two branches, and the result will be ugly
+at best (`git annex fix` can fix up the mess somewhat).
+
+Again, tuned repositories are an experimental feature; use with caution!
+
+The following tuning parameters are available:
+
+* `annex.tune.objecthashdirectories` (default: 2)  
+  Sets the number of hash directories to use in `.git/annex/objects/`
+
+* `annex.tune.objecthashlower` (default: false)
+  Set to true to make the hash directories in `.git/annex/objects/` use
+  all lower-case.
+
+* `annex.tune.branchhashdirectories` (default: 2)
+  Sets the number of hash directories to use in the git-annex branch.
+
+Note that git-annex will automatically propigate these setting to
+`.git/config` for tuned repsitories. You should never directly change
+these settings in `.git/config`