mailmap: use higher level string list functions

No functional changes intended. This commit makes use of higher level
and better documented functions of the string list API, so the code is
more understandable.

Note that also the required computational amount should not change
in principal as we need to look up the item no matter if it is already
part of the list or not. Once looked up, insertion comes for free.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
string_list: remove string_list_insert_at_index() from its API

There no longer is a caller to this function.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>