string-list: change "string_list_find_insert_index" return type to "size_t"

string-list: change "string_list_find_insert_index" return type to "size_t"

As "string_list_find_insert_index" is a simple wrapper of
"get_entry_index" and the return type of "get_entry_index" is already
"size_t", we could simply change its return type to "size_t".

Update all callers to use size_t variables for storing the return value.
The tricky fix is the loop condition in "mailmap.c" to properly handle
"size_t" underflow by changing from `0 <= --i` to `i--`.

Remove "DISABLE_SIGN_COMPARE_WARNINGS" from "mailmap.c" as it's no
longer needed with the proper unsigned types.

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