merge-recursive: fix some overly long lines

merge-recursive: fix some overly long lines

No substantive code change, just add some line breaks to fix lines that
have grown in length due to various refactorings.  Most remaining lines
of excessive length in merge-recursive include error messages and it's
not clear that splitting those improves things.

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