git-help: add "help.format" config variable.

git-help: add "help.format" config variable.

This config variable makes it possible to choose the default format
used to display help. This format will be used only if no option
like -a|--all|-i|--info|-m|--man|-w|--web is passed to "git-help".

The following values are possible for this variable:

	- "man"  --> "man" program is used
	- "info" --> "info" program is used
	- "web"  --> "git-browse-help" is used

By default we still show help using "man".

This patch also adds -m|--man command line option to use "man"
to allow overriding the "help.format" configuration variable.

Note that this patch also revert some recent changes in
"git-browse-help" because they prevented to look for config
variables in the global configuration file.

Signed-off-by: Christian Couder <chriscool@tuxfamily.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>