receive-pack: new config receive.procReceiveRefs

receive-pack: new config receive.procReceiveRefs

Add a new multi-valued config variable "receive.procReceiveRefs"
for `receive-pack` command, like the follows:

    git config --system --add receive.procReceiveRefs refs/for
    git config --system --add receive.procReceiveRefs refs/drafts

If the specific prefix strings given by the config variables match the
reference names of the commands which are sent from git client to
`receive-pack`, these commands will be executed by an external hook
(named "proc-receive"), instead of the internal `execute_commands`
function.

For example, if it is set to "refs/for", pushing to a reference such as
"refs/for/master" will not create or update reference "refs/for/master",
but may create or update a pull request directly by running the hook
"proc-receive".

Optional modifiers can be provided in the beginning of the value to
filter commands for specific actions: create (a), modify (m),
delete (d). A `!` can be included in the modifiers to negate the
reference prefix entry. E.g.:

    git config --system --add receive.procReceiveRefs ad:refs/heads
    git config --system --add receive.procReceiveRefs !:refs/heads

Signed-off-by: Jiang Xin <zhiyou.jx@alibaba-inc.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>