RFC proposal: set git defaults options from config

Mechanism
=========

I propose the following mechanism for setting default command line
options from the config:

options.<cmd> = <value>

is a "multivar" in git-config speak, i.e. it can appear multiple times.
When running "git <cmd> <opts>", our wrapper executes

git <cmd> <values> <opt>

where <values> is determined by the following rule in pseudocode:

if $GIT_OPTIONS_<cmd> is unset:
  <values> := empty
else:
  for <value> in $(git config --get-all options.cmd):
    if <value> matches the regexp in $GIT_OPTIONS_<CMD>:
      append <value> to <values>

Examples
========

* By default, no options can be overriden from config (other than those
which have config vars already, of course).

* A script which wants to protect options "foo" and "bar" of "cmd" from
being set by config sets GIT_OPTIONS_CMD="!(foo|bar)".

* A script which wants to allow overriding options "foo" and "bar" of
"cmd" by config (but nothing else) sets GIT_OPTIONS_CMD="foo|bar"

NOTES
=====

* This can be done by commit_pager_choice() or by a call right after
that in those places.
* regexp notation/version to be decided
* We should probably do this for long options only (and insert
"--<value>" rather than "<value>" to spare the "--" in config).
* We should probably do a prefix match.
* We could use GIT_OPTIONS_<CMD>_ALLOW and GIT_OPTIONS_<CMD>_DENY rather
than rely on negated regexps (if DENY matches deny, otherwise if ALLOW
matches allow, otherwise deny).
* We can get rid of a few config vars then...and may need to clean up
our option names.

Taking cover...

Michael