merge: narrow scope of merge_names

merge: narrow scope of merge_names

In order to pass the list of parents to fmt_merge_msg(), cmd_merge()
uses this strbuf to create something that look like FETCH_HEAD that
describes commits that are being merged.  This is necessary only
when we are creating the merge commit message ourselves, but was
done unconditionally.

Move the variable and the logic to populate it to confine them in a
block that needs them.

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