hg-update: add page

pages/common/hg-update.md

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+# hg update
+
+> Update the working directory to a specified changeset.
+
+- Update to the tip of the current branch:
+
+`hg update`
+
+- Update to the specified revision:
+
+`hg update --rev {{revision}}`
+
+- Update and discard uncommitted changes:
+
+`hg update --clean`
+
+- Update to the last commit matching a specified date:
+
+`hg update --date {{date}}`