Bug 1290201 - Don't clobber .mozbuild directory by default; r=ted

.mozbuild/last_log.json is opened by the process doing the deleting.
On Windows, you can't delete an opened file.

So stop clobbering the .mozbuild directory.

This directory only holds non-essential artifacts from mach/build
invocations. I don't think keeping it around will break anything.

MozReview-Commit-ID: 9b0AC2ywAAZ

--HG--
extra : rebase_source : 9236e3e282d1db2595e8b36353008bc98e2ae6cf

python/mozbuild/mozbuild/base.py

@@ -308,6 +308,7 @@ class MozbuildObject(ProcessExecutionMixin):
         """
         # Top-level files and directories to not clobber by default.
         no_clobber = {
+            '.mozbuild',
             'msvc',
         }