run-command-on-git-revisions aborts if run on a dirty repo

It would not be nice to run the run-command-on-git-revisions on a dirty repo. I added a check with some hints from [stackoverflow](http://stackoverflow.com/a/2659808). The script now aborts if you have staged or unstaged changes or even untracked files.

Maybe I could add a --force flag to run anyway?
fixed whitespace and function name in run-command-on-git-revisions
run-command-on-git-revisions prints error messages

The script now actually prints error messages when it runs on a dirty
repository.

"$@" instead of $* is more robust with respect to quoting whitespace in
arguments,
    see [this stackoverflow answer](http://stackoverflow.com/a/9915672)
comparing exit codes in run-command-on-git-revisions
Merge remote-tracking branch 'Enucatl/patch-1'
whitespace
remove superfluous echo