Guitar - local changes would be overwritten by merge

[kofalt]

I occasionally run into this problem with existing graphs. Pretty sure it would be fixed by #53 but tracking it separately as it's a user-visible error.

Building from  to index.docker.io/ubuntu/14.04
Opening destination repository
...
Committing to graph...
Starting publish of  index.docker.io/ubuntu/14.04  <--  
Lineage already existed.
Exporting files...
Exporting metadata...
error: Your local changes to the following files would be overwritten by merge:
    .guitar
Please, commit your changes or stash them before you can merge.
Aborting
gosh: command "git" exited with unexpected status 128

Hroot crashed! This could be a problem with docker or git, or hroot itself.