string-list: return index directly when inserting an existing element

string-list: return index directly when inserting an existing element

When inserting an existing element, "add_entry" would convert "index"
value to "-1-index" to indicate the caller that this element is in the
list already. However, in "string_list_insert", we would simply convert
this to the original positive index without any further action.

In 8fd2cb4069 (Extract helper bits from c-merge-recursive work,
2006-07-25), we create "path-list.c" and then introduce above code path.

Let's directly return the index as we don't care about whether the
element is in the list by using "add_entry". In the future, if we want
to let "add_entry" tell the caller, we may add "int *exact_match"
parameter to "add_entry" instead of converting the index to negative to
indicate.

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