git-fmt-merge-msg: make it usable from other builtins

git-fmt-merge-msg: make it usable from other builtins

Move all functionality (except config and option parsing) from
cmd_fmt_merge_msg() to fmt_merge_msg(), so that other builtins can use
it without a child process.

All functions have been changed to use strbufs, and now only
cmd_fmt_merge_msg() reads directly from a file / writes anything to
stdout.

Signed-off-by: Miklos Vajna <redacted>
Signed-off-by: Junio C Hamano <redacted>