Make magit-save-buffers-predicate-tree-only more robust

This is similar to the previous commit.  String-prefix-p is a fragile
way to determine if a file is contained in a subdirectory.
(`file-in-directory-p' is normally the right thing to use for that, but
it will hit tramp).  Also see #1545.
Update mastering-intro variable to a not dead link.
Merge pull request #1552 from pskrz/master

Update mastering-intro variable to a not dead link.
revert all recent changes to magit-revert-buffers

This reverts
  140e8239eccb45a5673c770d1f56c01e921ecceb
  febc832cbe12739309c41e453b36067dd4c4bc55
  68af16a2cf1d3a31e90beea80aa97f5ecc9c797e

These changes have lead to a performance regression and while some users
bothered to report it or tweet about it I have so far gotten little help
in the form of replies to my questions or reliable benchmarks.

See #1544, #1545 and #1546.

Note that this reverts changes to functionality which would be
desirable.  Generally I no longer make any functional changes to the
master branch and instead concentrate getting the next branch ready
for merge into master.  I have made an exception here and it bit me.
README.md: mention release has priority over proposed features

Also add a very prominent link to the FAQ.
README.md: fix typo
Merge branch 'master' into next

Except for a fixed link in `README.md' discard all changes.  Other
changes to the `README.md' will be incorporated once they have been
improved some more.  The changes to `magit.el' are not relevant on
`next', they are reverts of changes which also were discarded when
they were merged.

Conflicts:
	magit.el