diffcore-rename: avoid usage of global in too_many_rename_candidates()

diffcore-rename: avoid usage of global in too_many_rename_candidates()

too_many_rename_candidates() got the number of rename destinations via
an argument to the function, but the number of rename sources via a
global variable.  That felt rather inconsistent.  Pass in the number of
rename sources as an argument as well.

While we are at it... We had a local variable, num_src, that served two
purposes.  Initially it was set to the global value, but later was used
for counting a subset of the number of sources.  Since we now have a
function argument for the former usage, introduce a clearer variable
name for the latter usage.

This patch has no behavioral changes; it's just renaming and passing an
argument instead of grabbing it from the global namespace.  (You may
find it easier to view the patch using git diff's --color-words option.)

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