revert: check for changes against target rev

pick a non-conflicting module name when loading an extension prefix the module name by 'hgext_' so that it doesn't conflict with any core python module (for example bisect -> hgext_bisect) fix issue271

Fixed 'hg serve --webdir-conf foo' which broke due to split of hgweb.

Further cleanup of ui.py (changeset 74d569332f8b used one-char variable names).

packagescan can't scan nested packages

move purge extension out of hgext. it is not as clean as other modules in hgext. can move back after rewrite.

import: allow to import stdin with file name "-"

fix comment.