doc: interpret-trailers: don’t use deprecated config

`command` has been deprecated since commit c364b7e (trailer: add new
.cmd config option, 2021-05-03).

Use the commit message of c364b7e as a guide to replace the use of
`$ARG` and to use a script instead of an inline command.[1] Also,
explicitly trigger the command by passing in `--trailer=see`, since
this config is not automatically used.[2]

[1]: “Instead of "$ARG", users can refer to the value as positional
   argument, $1, in their scripts.”
[2]: “At the same time, in order to allow `git interpret-trailers` to
   better simulate the behavior of `git command -s`,
   'trailer.<token>.cmd' will not automatically execute.”

Acked-by: ZheNing Hu <[email protected]>
Signed-off-by: Kristoffer Haugsbakk <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: LemmingAvalanche

Documentation/git-interpret-trailers.txt

@@ -325,7 +325,7 @@ $ git interpret-trailers --trailer 'Cc: Alice <[email protected]>' --trailer 'Re
 $ git config trailer.sign.key "Signed-off-by: "
 $ git config trailer.sign.ifmissing add
 $ git config trailer.sign.ifexists doNothing
-$ git config trailer.sign.command 'echo "$(git config user.name) <$(git config user.email)>"'
+$ git config trailer.sign.cmd 'echo "$(git config user.name) <$(git config user.email)>"'
 $ git interpret-trailers </dev/null
 
 Signed-off-by: Bob <[email protected]>
@@ -405,11 +405,14 @@ subject
 message
 
 see: HEAD~2
+$ cat ~/bin/glog-ref
+#!/bin/sh
+git log -1 --oneline --format="%h (%s)" --abbrev-commit --abbrev=14
 $ git config trailer.see.key "See-also: "
 $ git config trailer.see.ifExists "replace"
 $ git config trailer.see.ifMissing "doNothing"
-$ git config trailer.see.command "git log -1 --oneline --format=\"%h (%s)\" --abbrev-commit --abbrev=14 \$ARG"
-$ git interpret-trailers <msg.txt
+$ git config trailer.see.cmd "glog-ref"
+$ git interpret-trailers --trailer=see <msg.txt
 subject
 
 message