This option can be used to specify a shell command that will be called:

once to automatically add a trailer with the specified <token>, and then

each time a '--trailer <token>=<value>' argument to modify the <value> of

the trailer that this option would produce.

When the specified command is first called to add a trailer

with the specified <token>, the behavior is as if a special

'--trailer <token>=<value>' argument was added at the beginning

of the "git interpret-trailers" command, where <value>

is taken to be the standard output of the command with any

leading and trailing whitespace trimmed off.

If some '--trailer <token>=<value>' arguments are also passed

on the command line, the command is called again once for each

of these arguments with the same <token>. And the <value> part

of these arguments, if any, will be used to replace the first

occurrence of substring `$ARG` in the command. This way the

command can produce a <value> computed from the <value> passed

in the '--trailer <token>=<value>' argument.

For consistency, the first occurrence of substring `$ARG` is

also replaced, this time with the empty string, in the command

when the command is first called to add a trailer with the

specified <token>.