push options: {pre,post}-receive hook learns about push options

The environment variable GIT_PUSH_OPTION_COUNT is set to the number of
push options sent, and GIT_PUSH_OPTION_{0,1,..} is set to the transmitted
option.

The code is not executed as the push options are set to NULL, nor is the
new capability advertised.

There was some discussion back and forth how to present these push options
to the user as there are some ways to do it:

Keep all options in one environment variable
============================================
+ easiest way to implement in Git
- This would make things hard to parse correctly in the hook.

Put the options in files instead,
filenames are in GIT_PUSH_OPTION_FILES
======================================
+ After a discussion about environment variables and shells, we may not
  want to put user data into an environment variable (see [1] for example).
+ We could transmit binaries, i.e. we're not bound to C strings as
  we are when using environment variables to the user.
+ Maybe easier to parse than constructing environment variable names
  GIT_PUSH_OPTION_{0,1,..} yourself
- cleanup of the temporary files is hard to do reliably
- we have race conditions with multiple clients pushing, hence we'd need
  to use mkstemp. That's not too bad, but still.

Use environment variables, but restrict to key/value pairs
==========================================================
(When the user pushes a push option `foo=bar`, we'd
GIT_PUSH_OPTION_foo=bar)
+ very easy to parse for a simple model of push options
- it's not sufficient for more elaborate models, e.g.
  it doesn't allow doubles (e.g. cc=reviewer@email)

Present the options in different environment variables
======================================================
(This is implemented)
* harder to parse as a user, but we have a sample hook for that.
- doesn't allow binary files
+ allows the same option twice, i.e. is not restrictive about
  options, except for binary files.
+ doesn't clutter a remote directory with (possibly stale)
  temporary files

As we first want to focus on getting simple strings to work
reliably, we go with the last option for now. If we want to
do transmission of binaries later, we can just attach a
'side-channel', e.g. "any push option that contains a '\0' is
put into a file instead of the environment variable and we'd
have new GIT_PUSH_OPTION_FILES, GIT_PUSH_OPTION_FILENAME_{0,1,..}
environment variables".

[1] 'Shellshock' https://lwn.net/Articles/614218/

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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