[GitSVN][NFC] Mark dry-run commits as such in the log output

Summary: This helps to avoid worries about the "dry run flag" while testing.

Reviewers: jyknight, rnk, mehdi_amini

Subscribers: bollu, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64697

llvm-svn: 366023