doc: make gendoc.py module import policy aware
Without this, running gendoc.py during an install without C modules
available (via `make local`) will result in an import failure because
the default module load policy insists on C modules.
We also remove the sys.path adjustment because it is no longer needed
since our magic importer handles things.
Test update logic when there are renames
Update with local changes across a file rename
$ hg init
$ echo a > a
$ hg add a
$ hg ci -m a
$ hg mv a b
$ hg ci -m rename
$ echo b > b
$ hg ci -m change
$ hg up -q 0
$ echo c > a
$ hg up
merging a and b to b
warning: conflicts while merging b! (edit, then use 'hg resolve --mark')
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
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