magit-revision-insert-related-refs: new variable

This is intended as a temporary solution to the problem that `git branch
--contains` and `git branch --merge` can be very slow when there are
many branches.  This variable will likely be replaced by a `git-config'
variable, once Magit starts using such variables for "repository-local
variables".  This fixes #1630.
magit-insert-untracked-files: respect status.showUntrackedFiles

Showing all untracked files can be slow.  So respect the Git variable
`status.showUntrackedFiles' allowing users to make it faster at the
cost of not being able to stage every untracked file individually.

That variable defaults to "normal" which means that when directory
contains nothing but untracked files then only the directory is shown.

Before respecting this variable Magit showed each untracked files using
an initially collapsed tree.  Doing that would probably be the more
reasonable default, but I will delay the final decision until after the
following and just do what Git would do for now.

In the future we might use `magit-status.showUntrackedFiles' instead of
sharing `status.showUntrackedFiles' with Git.  Then we could also have
another default ("all") without anyone being able to say that doing so
was wrong.
Makefile: remove install-all target
remove obsolete manual

The new manual will contain text copied from Git's documentation.

The old manual was licensed under the FDL v1.2, while Git's
documentation is licensed under the GPL v2.  Being able to seed Magit's
documentation with pieces from Git is more valuable than keeping pieces
from Magit's old documentation (most of it would have to be discarded
anyway) and so the latter has to go.
README.md: indent code blocks

Don't use ```language ...```.  That looks worse in plain text
and while it used to look better on Github, it no longer does.

See http://www.greghendershott.com/2014/11/github-dropped-pygments.html.