Fix tag builds after the more-complete `isort` edit. (#4532)
Fix tag builds after the more-complete `isort` edit. (#4532)

### Problem

94775d9 switched `build-support/bin/isort.sh` to a more useful range for `--changed`: from `master` to `HEAD`. Unfortunately, when travis builds git tags, it does so in a shallow clone that does not contain master.

### Solution

Detect whether we're in a partial clone, and skip isort checks. It's possible that there is a way to get travis to always include master while cloning for tags, but I couldn't find it.

### Result

https://travis-ci.org/pantsbuild/pants/builds/227030779 should be fixable.
baroquebobcat/pants,tdyas/pants,fkorotkov/pants,UnrememberMe/pants,twitter/pants,foursquare/pants,UnrememberMe/pants,tdyas/pants,15Dkatz/pants,wisechengyi/pants,baroquebobcat/pants,landism/pants,wisechengyi/pants,landism/pants,foursquare/pants,benjyw/pants,pantsbuild/pants,pombredanne/pants,pombredanne/pants,fkorotkov/pants,landism/pants,pombredanne/pants,twitter/pants,jsirois/pants,baroquebobcat/pants,fkorotkov/pants,UnrememberMe/pants,15Dkatz/pants,fkorotkov/pants,landism/pants,twitter/pants,foursquare/pants,fkorotkov/pants,twitter/pants,benjyw/pants,benjyw/pants,wisechengyi/pants,15Dkatz/pants,fkorotkov/pants,tdyas/pants,tdyas/pants,UnrememberMe/pants,benjyw/pants,15Dkatz/pants,landism/pants,tdyas/pants,baroquebobcat/pants,benjyw/pants,fkorotkov/pants,UnrememberMe/pants,tdyas/pants,benjyw/pants,landism/pants,tdyas/pants,wisechengyi/pants,foursquare/pants,pantsbuild/pants,fkorotkov/pants,baroquebobcat/pants,pombredanne/pants,foursquare/pants,15Dkatz/pants,twitter/pants,pombredanne/pants,tdyas/pants,wisechengyi/pants,pombredanne/pants,pombredanne/pants,twitter/pants,foursquare/pants,benjyw/pants,foursquare/pants,baroquebobcat/pants,wisechengyi/pants,twitter/pants,UnrememberMe/pants,baroquebobcat/pants,baroquebobcat/pants,baroquebobcat/pants,pantsbuild/pants,pantsbuild/pants,UnrememberMe/pants,foursquare/pants,pantsbuild/pants,jsirois/pants,twitter/pants,wisechengyi/pants,pantsbuild/pants,twitter/pants,tdyas/pants,UnrememberMe/pants,jsirois/pants,pantsbuild/pants,15Dkatz/pants,foursquare/pants,wisechengyi/pants,landism/pants,landism/pants,wisechengyi/pants,UnrememberMe/pants,15Dkatz/pants,15Dkatz/pants