Start of hotfix development
Fixes some environment variabel names
Fix variable non-zero test

Due to missing quotation marks, the -n test won't work properly, leading
to (strangely) always returning true and overriding variables with
nothing.

That causes "release finish" and "hotfix finish" to silently ignore the
flags -u, -m, and -f, and perhaps also some others which I didn't try.

With the quotation marks in place now that shouldn't be an issue
anymore.
Provide an environmental var for --showcommands
Small typo in the changelog
Version bump 1.5.1
Merge branch 'hotfix/1.5.1'
Merge tag '1.5.1' into develop

v1.5.1
Version bump 1.6.0