tests/test-module-imports.t

rebase: improve error message for --base being empty or causing emptiness

Before it just said 'nothing to rebase'.

Now 'if "base" is an empty set:
  abort: empty "base" revision set - can't compute rebase set

If the set of changesets to rebase can't be found from "base", it will fail as
before but with more explanation of what the problem was.

The name of the "base" option is not obvious - it is more like "samples
identifying the branch to rebase". The error messages for problems with the
specified "base" value will use that term and might thus also not be obvious,
but at least they are consistent with the option name. The name "base" will not
be used if the base only was specified implicitly as the working directory
parent.

This code uses the ast module, which was new in 2.6, so we'll skip
this test on anything earlier.
  $ python -c 'import sys ; assert sys.version_info >= (2, 6)' || exit 80

  $ import_checker="$TESTDIR"/../contrib/import-checker.py
Run the doctests from the import checker, and make sure
it's working correctly.
  $ TERM=dumb
  $ export TERM
  $ python -m doctest $import_checker

  $ cd "$TESTDIR"/..
  $ if hg identify -q > /dev/null 2>&1; then :
  > else
  >     echo "skipped: not a Mercurial working dir" >&2
  >     exit 80
  > fi

There are a handful of cases here that require renaming a module so it
doesn't overlap with a stdlib module name. There are also some cycles
here that we should still endeavor to fix, and some cycles will be
hidden by deduplication algorithm in the cycle detector, so fixing
these may expose other cycles.

  $ hg locate 'mercurial/**.py' | xargs python "$import_checker"
  mercurial/dispatch.py mixed stdlib and relative imports:
     commands, error, extensions, fancyopts, hg, hook, util
  mercurial/fileset.py mixed stdlib and relative imports:
     error, merge, parser, util
  mercurial/revset.py mixed stdlib and relative imports:
     discovery, error, hbisect, parser, phases, util
  mercurial/templater.py mixed stdlib and relative imports:
     config, error, parser, templatefilters, util
  mercurial/ui.py mixed stdlib and relative imports:
     config, error, formatter, scmutil, util
  Import cycle: mercurial.cmdutil -> mercurial.subrepo -> mercurial.cmdutil
  Import cycle: mercurial.repoview -> mercurial.revset -> mercurial.repoview
  Import cycle: mercurial.fileset -> mercurial.merge -> mercurial.subrepo -> mercurial.match -> mercurial.fileset
  Import cycle: mercurial.filemerge -> mercurial.match -> mercurial.fileset -> mercurial.merge -> mercurial.filemerge