Ignore excluded files as early as possible in Svn2Infill2Processor

Created as a follow-up of https://fisheye.dev.internal.atlassian.com/cru/CR-FE-9776#c101109.

When collecting data about an add changelist at the root level in Svn2Infill2Processor#summarizeRootDiff, we must immediately ignore the excluded files from Svn2ChangesetDiffSummary. This became apparent when we fixed importing files from outside the repo in FE-6274. This resulted in redundant svn cat and svn proplist calls for excluded files that were later on ignored anyway.