trailer: append trailers without fork/exec

trailer: append trailers without fork/exec

Introduce amend_strbuf_with_trailers() to apply trailer additions to a
message buffer via process_trailers(), avoiding the need to run git
interpret-trailers as a child process.

Update amend_file_with_trailers() to use the in-process helper and
rewrite the target file via tempfile+rename, preserving the previous
in-place semantics. As the trailers are no longer added in a separate
process and trailer_config_init() die()s on missing config values it
is called early on in cmd_commit() and cmd_tag() so that they die()
early before writing the message file. The trailer arguments are now
also sanity checked.

Keep existing callers unchanged by continuing to accept argv-style
--trailer=<trailer> entries and stripping the prefix before feeding the
in-process implementation.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Phillip Wood <phillip.wood@dunelm.org.uk>
Signed-off-by: Junio C Hamano <gitster@pobox.com>