builtin/merge: avoid -Wformat-extra-args from ancient Xcode

builtin/merge: avoid -Wformat-extra-args from ancient Xcode

d540b70c85 (merge: cleanup messages like commit, 2019-04-17) adds
a way to change part of the helper text using a single call to
strbuf_add_commented_addf but with two formats with varying number
of parameters.

this trigger a warning in old versions of Xcode (ex 8.0), so use
instead two independent calls with a matching number of parameters

Signed-off-by: Carlo Marcelo Arenas Belón <redacted>
Signed-off-by: Junio C Hamano <redacted>