MDLSITE-3608 php_lint: fix error handling on difference compairsons

Previous error handling relied on non-strict error checking, so when the list_changed_files.sh died without useful output, this script died without useful output.

Now it'll default back to a full lint as desired.

You can test it with this:
```
#!/bin/bash
set -e

export gitcmd=/usr/local/bin/git
export phpcmd=/usr/local/bin/php
export gitdir=/Users/danp/www/im
# Set to this to see it do full lint:
export GIT_PREVIOUS_COMMIT=random-junk-or-whatever
# To do incremental:
export GIT_PREVIOUS_COMMIT=293f1b4d85d5a9b4b22b8e37eb2283f9a0b04157
export GIT_COMMIT=3d4de122f20830405b3e0d0f653c88851e5e2777

./php_lint.sh
```