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).
$ hg init
$ echo a > a
$ hg ci -qAm 'add a'
$ hg init subrepo
$ echo 'subrepo = http://example.net/libfoo' > .hgsub
$ hg ci -qAm 'added subrepo'
$ hg up -qC 0
$ echo ax > a
$ hg ci -m 'changed a'
created new head
$ hg up -qC 1
$ cd subrepo
$ echo b > b
$ hg add b
$ cd ..
Should fail, since there are added files to subrepo:
$ hg merge
abort: uncommitted changes in subrepository 'subrepo'
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