make reading passwords customizable

We use a process filter to detect when git or one of its subprocesses
(in many cases ssh) prompts the user for usernames, passwords and
passphrases.  Until now the prompt strings we were able to detect were
hardcoded.

Add new options magit-process-{username,password}-prompts so that users
can easily tell magit to forward the (sometimes quite unique) prompts
their setup spits out, without having to patiently having to wait for us
maintainers to agree, to support them.
magit-process-{username,password}-prompts: pimp regexps
forward git/ssh yes-or-no prompts to the user
Merge branch 'prompt'
remove magit-bug-report

I do not think having such a function is useless per se but in its
current form it is.

1. I have never actually seen a bug report that looked that it was based
   on the contents of buffer created by it.

2. It was only used in very few places and it appears these places never
   actually cause any errors.  (Actually in the case of `magit-run*'
   errors are less unlikely).

3. For this function to be useful it would have to be used everywhere
   errors can happen.  Doing so would be way to ugly without first
   converting it to a macro and then some.

4. Just writing some versions strings into a buffer and telling the user
   to do something is not particularly useful.  Something using Message
   mode might be, though.

So yes, maybe I will add back something similar, but it has to be
redesigned to be useful at all.  And there are more important things to
do first.
update modeline inside `magit-revert-buffer'

We have a mechanism for "updating buffers that need it": the refresh
mechanism implemented by the `magit-refresh-*' functions and macros.

Refreshing the modeline on behalf of vc did not make use of that but
instead relied on `magit-update-vc-modeline' being called explicitly.
Now it is refreshed whenever the regular refresh mechanism kicks in,
which can both positive or negative.

Positive because we don't have to identify each place where updating
the modeline would make sense; negative because it is a potentially
costly operation which is now done under circumstances that do not
necessarily call for it.

I will take care of the drawbacks by making this both less costly as
well as optional all together, in later commits.  (This commit already
makes refreshing slightly more efficient because it removes one looping
of the list of buffers.)

The name of this function now doesn't match all it does anymore.
That will also be taken care of.
magit-refresh-wrapper: cosmetics
only call dired-uncache when needed

When dired isn't loaded then there is no information to be uncashed.
Don't load dired just to unload a cache that doesn't even exist.
add magit-refresh-file-buffer-hook

Call it in `magit-refresh-wrapper' for every buffer visiting a file in
the current buffer.

Bring back `magit-update-vc-modeline' as a wrapper around
`vc-find-file-hook' to update _a single_ buffer.  Do _not_ add it to the
hook's default value.  I don't expect many people to pay attention to
the Vc controlled part of the modeline (and if they do they will notice
and can add it themselves).  On the other hand many people complain
about bad performance and not using this function does improve that.
See issue #573.  We should implement our own "status in modeline" hook
function, see issue #240.

Add new function `magit-revert-buffer' to the hook.  It does the same
thing as the now removed `magit-revert-buffers' (plural) did until a
four commits ago - just for a single buffer.
Merge branch 'refresh'