cmdutil.tryimportone: allow importing relative patches into the working dir This makes hg import --prefix dir/ (without bypass) work.

commands.import: accept a prefix option The --prefix option is meant to be relative to the root rather than the current working directory. This is for consistency with the rest of 'hg import' -- paths in patches are otherwise considered to be relative to the root. In upcoming patches we'll hook this option up to the patch functions. --exact with --prefix is currently disallowed because I can't really come up with sensible semantics for it, especially when only part of the patch is preserved.

test: make version based test-extensions failure more explanatory A buggy __version__ content (usually from setup.py's hg being unable to read the repo) can make this test fails in an obscure manner. We make the root of the failure more explicit.