extensions: list up only enabled extensions, if "ui" is specified

Before this patch, "extensions.extensions()" always lists up all
loaded extensions. So, commands handling multiple repositories at a
time like below enable extensions unexpectedly.

  - clone from or push to localhost: extensions enabled only in the
    source are enabled also in the destination

  - pull from localhost: extensions enabled only in the destination
    are enabled also in the source

  - recursive execution in subrepo tree: extensions enabled only in
    the parent or some of siblings in the tree are enabled also in
    others

In addition to it, extensions disabled locally may be enabled
unexpectedly.

This patch checks whether each of extensions should be listed up or
not, if "ui" is specified to "extensions.extensions()", and invokes
"reposetup()" of each extensions only for repositories enabling it.
localrepo: make supported features manageable in each repositories individually

Before this patch, all localrepositories support same features,
because supported features are managed by the class variable
"supported" of "localrepository".

For example, "largefiles" feature provided by largefiles extension is
recognized as supported, by adding the feature name to "supported" of
"localrepository".

So, commands handling multiple repositories at a time like below
misunderstand that such features are supported also in repositories
not enabling corresponded extensions:

  - clone/pull from or push to localhost
  - recursive execution in subrepo tree

"reposetup()" can't be used to fix this problem, because it is invoked
after checking whether supported features satisfy ones required in the
target repository.

So, this patch adds the set object named as "featuresetupfuncs" to
"localrepository" to manage hook functions to setup supported features
of each repositories.

If any functions are added to "featuresetupfuncs", they are invoked,
and information about supported features is managed in each
repositories individually.

This patch also adds checking below:

  - pull from localhost: whether features supported in the local(= dst)
    repository satisfies ones required in the remote(= src)

  - push to localhost: whether features supported in the remote(= dst)
    repository satisfies ones required in the local(= src)

Managing supported features by the class variable means that there is
no difference of supported features between each instances of
"localrepository" in the same Python process, so such checking is not
needed before this patch.

Even with this patch, if intermediate bundlefile is used as pulling
source, pulling indirectly from the remote repository, which requires
features more than ones supported in the local, can't be prevented,
because bundlefile has no information about "required features" in it.
largefiles: setup "largefiles" feature in each repositories individually

Before this patch, if largefiles extension is enabled once in any of
target repositories, commands handling multiple repositories at a time
like below misunderstand that "largefiles" feature is supported also
in all other local repositories:

  - clone/pull from or push to localhost
  - recursive execution in subrepo tree

This patch registers "featuresetup()" into "featuresetupfuncs" of
"localrepository" to support "largefiles" features only in
repositories enabling largefiles extension, instead of adding
"largefiles" feature to class variable "_basesupported" of
"localrepository".

This patch also adds checking below to the largefiles specific class
derived from "localrepository":

  - push to localhost: whether features supported in the local(= dst)
    repository satisfies ones required in the remote(= src)

This can prevent useless looking up in the remote repository, when
supported and required features are mismatched: "push()" of
"localrepository" also checks it, but it is executed after looking up
in the remote.
hgweb: add reset javascript function to Graph

It makes the Graph object to be in the same state as just after
the initialization. This will help to add infinite scrolling to graph view.
hgweb: make infinite scroll handling more generic and extensible

Namely, this allows the next page pointer to be not only revision hash given
in page code, but also any value computed from the value for previous page.
hgweb: add graph mode of ajax response processing

While the default mode appends all the new entries to a container on the page,
the graph mode resizes canvas correctly, and repaints the graph to include
newly received data.
paper: add infinite scrolling to graph by calling ajaxScrollInit at the page
tests: fix check-code breakage

--HG--
branch : stable
merge with stable