Raise error if there are two spec files
merge common code
Do not create patch if there are binary files

"git diff" can accept --binary, which will produce diff with binary content. But patch does not understood
such diff, so we are doomed. Diff with binary file will produce

Binary files /dev/null and b/app/assets/images/false.png differ
diff --git a/app/assets/images/foreman.png b/app/assets/images/foreman.png
new file mode 100644
index 0000000..8815824

And patch will accept such syntax but will not create the file. Not even empty one.
So this result in silently throwing away some changes.
Merge pull request #66 from xsuchy/pull-req-binary

Do not create patch if there are binary files
Merge pull request #65 from xsuchy/pull-req-spec2

Fail if there are two spec files in directory