merge-recursive: add pointer about unduly complex looking code

merge-recursive: add pointer about unduly complex looking code

handle_change_delete() has a block of code displaying one of four nearly
identical messages.  Each contains about half a dozen variable
interpolations, which use nearly identical variables as well.  Someone
trying to parse this may be slowed down trying to parse the differences
and why they are here; help them out by adding a comment explaining the
differences.

Further, point out that this code structure isn't collapsed into something
more concise and readable for the programmer, because we want to keep full
messages intact in order to make translators' jobs much easier.

Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>