hghave: add "absimport" feature to check "absolute_import" in __future__

This patch adds "absimport" feature to check whether "absolute_import"
exists in __future__, which means supporting module loading by
absolute name.

This check is needed for portability of test code using
"absolute_import", because Python earlier than 2.5 doesn't support it.
hghave: add "py3k" feature to check whether test runs with Python 3.x

This patch adds "py3k" feature to check whether test runs with Python
3.x.

This check is needed for portability of test code: for example, in the
default, modules are imported relatively first with python 2.x, but
imported absolutely with Python 3.x.
demandimport: support "absolute_import" for external libraries (issue4029)

Before this patch, demandimport of Mercurial may fail to load external
libraries using "from __future__ import absolute_import": for example,
importing "foo" in "bar.baz" module will load "bar.foo" if it exists,
even though "absolute_import" is enabled in "bar.baz" module.

So, extensions for Mercurial can't use such external libraries.

This patch saves "level" of import request for on-demand module
loading in the future: default value of level is -1, and level is 0
when "absolute_import" is enabled.

"level" value is passed to built-in import function in
"_demandmod._load()" and it should load target module correctly.

This patch changes only one "_demandmod" construction case other than
cases below:

    - construction in "_demandmod._load()"

      this code path should be used only in relative sub-module
      loading case

    - constructions other than patched one in"_demandimport()"

      these code paths shouldn't be used in "level != -1" case
demandimport: allow extensions to import own modules by absolute name

Before this patch, python modules of each extensions can't import
another one in own extension by absolute name, because root modules of
each extensions are loaded with "hgext_" prefix.

For example, "import extroot.bar" in "extroot/foo.py" of "extroot"
extension fails, even though "import bar" in it succeeds.

Installing extensions into site-packages of python library path can
avoid this problem, but this solution is not reasonable in some cases:
using binary package of Mercurial on Windows, for example.

This patch retries to import with "hgext_" prefix after ImportError,
if the module in the extension may try to import another one in own
extension.

This patch doesn't change some "_import()"/"_origimport()" invocations
below, because ordinary extensions shouldn't cause such invocations.

    - invocation of "_import()" when root module imports sub-module by
      absolute path without "fromlist"

      for example, "import a.b" in "a.__init__.py".

      extensions are loaded with "hgext_" prefix, and this causes
      execution of another (= fixed by this patch) code path.

    - invocation of "_origimport()" when "level != -1" with "fromlist"

      for example, importing after "from __future__ import
      absolute_import" (level == 0), or "from . import b" or "from .a
      import b" (0 < level),

      for portability between python versions and environments,
      extensions shouldn't cause "level != -1".
push: hide description about "-f" in the hint to prevent from using it easily

"use push -f to force" in the hint at abortion of "hg push" may cause
novice users to execute "push -f" easily without understanding about
problems of multiple branch heads in the repository.

This patch hides description about "-f" in the hint, and leads into
seeing "hg help push" for details about pushing new heads.
push: add more detailed explanation about "--force" to online help document

This patch adds more detailed explanation about "--force" to online
help document of "hg push" to prevent novice users to execute "push
--force" easily without understanding about problems of multiple
branch heads in the repository.
discovery: revise hint message introduced by changeset b00ba31313c3

This patch revises hint message from "for detail about" introduced by
changeset b00ba31313c3 to "for details about", to unify it with the
hint message introduced by proceeding patch.