Dump problematic parser patches to temp files in "arc diff"

Summary: I'm trying to get a repro for a Windows + SVN patch issue. Dump patches which fail to a temp file so there's less bewilderment in getting the right patch handed over for analysis.

Test Plan: Forced a parse failure, ran "arc diff", inspected temp file.

Reviewers: btrahan, vrana

Reviewed By: btrahan

CC: aran

Differential Revision: https://secure.phabricator.com/D2997