Fixes #178: Use dry-run to test patch to avoid leftover files/folders.

[pescetti]

Fixes https://github.com/cweagans/composer-patches/issues/178 for branch 1.x.

[danepowell]

I've tested and confirmed that this fixes the problem of extraneous folders getting left around following composer updates, and it has no side effects / regressions. Is there anything holding up merging this?

Environments tested:

• Ubuntu 18.04, Git 2.17.0, Composer 1.6.4
• MacOS 10.13.4, Git 2.17.0, Composer 1.6.4
• Debian 8.10, Git 2.1.4, Composer 1.6.3

[prudloff-insite]

I also confirm that this fixes our problem (tested with Composer 2.0.6 on a large project with dozens of patches). Is there any chance that this will be merged?

[cweagans]

I've made a note to capture this in #447.