revlog: introduce isancestor method for efficiently determining node lineage Hide the not so obvious use of commonancestorsheads.

clone: provide sample username = config entry in .hg/hgrc (issue4359) Some users clone from a server before ever running 'hg config --edit', so they don't see our helpful template for things like enabling the username. Attempt to give them some helpful guidance.

test-acl: alter sed construct to avoid changes in .hg/hgrc formatting A future patch is going to add some extra commented-out boilerplate to the top of .hg/hgrc during clone. In order to make this test not require regular updates, switch to searching for [hooks] or [acl] and print file from the first match to that pattern.