add: pass full=False to dirstate walk
Previously cmdutil.add would call wctx.walk(), which under the hood calls
dirstate.walk with full=True. This means it returns all of the clean files
(which we don't need when computing the add set), as well as the unclean files.
This results in 1) a lot more work being done and 2) this code path
circumventing the hgwatchman extension, resulting in worse performance in
hgwatchman environments ('hg add .' went from 9s to 1.8s).
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