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.: