Add git-imerge

Created by Michael Haggerty <mhagger@alum.mit.edu>. Cloned from
git@github.com:mhagger/git-imerge.git and this version of git-imerge was
added to the repo in commit b26d9d3511aead4943967764e85af1a18782b33d
("Let NothingToDoError construct its own error message.", 2014-05-30
17:35:01 +0200).

581f01ea-27c1-11e4-9cbd-c80aa9e67bbd