merge-file: refactor for subsequent memory leak fix

merge-file: refactor for subsequent memory leak fix

Refactor the code in builtin/merge-file.c to:

 * Use the initializer to zero out "mmfs", and use modern C syntax for
   the rest.

 * Refactor the the inner loop to use a variable and "if/else if"
   pattern followed by "return". This will make a change to change it to
   a "goto cleanup" pattern smaller.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>