help: use "man.<tool>.cmd" as custom man viewer command

help: use "man.<tool>.cmd" as custom man viewer command

Currently "git help -m GITCMD" is restricted to a set of man viewers
defined at compile time. You can subvert the "man.<tool>.path" to
force "git help -m" to use a different man, viewer, but if you have a
man viewer whose invocation syntax does not match one of the current
tools then you would have to write a wrapper script for it.

This patch adds a git config variable "man.<tool>.cmd" which allows a
more flexible man viewer choice.

If you run "git help -m GITCMD" with the "man.viewer" config variable
set to an unrecognized tool then it will query the "man.<tool>.cmd"
config variable. If this variable exists, then the specified tool will
be treated as a custom man viewer and it will be run in a shell with
the man page name of the GITCMD added as extra parameter.

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