mailmap: remove email copy and length limitation

In map_user(), we have email pointer that points at the beginning of
an e-mail address, but the buffer is not terminated with a NUL after
the e-mail address.  It typically has ">" after the address, and it
could have even more if it comes from author/committer line in a
commit object.  Or it may not have ">" after it.

We used to copy the e-mail address proper into a temporary buffer
before asking the string-list API to find the e-mail address in the
mailmap, because string_list_lookup() function only takes a NUL
terminated full string.

Introduce a helper function lookup_prefix that takes the email
pointer and the length, and finds a matching entry in the string
list used for the mailmap, by doing the following:

 - First ask string_list_find_insert_index() where in its sorted
   list the e-mail address we have (including the possible trailing
   junk ">...") would be inserted.

 - It could find an exact match (e.g. we had a clean e-mail address
   without any trailing junk).  We can return the item in that case.

 - Or it could return the index of an item that sorts after the
   e-mail address we have.

 - If we did not find an exact match against a clean e-mail address,
   then the record we are looking for in the mailmap has to exist
   before the index returned by the function (i.e. "email>junk"
   always sorts later than "email").  Iterate, starting from that
   index, down the map->items[] array until we find the exact record
   we are looking for, or we see a record with a key that definitely
   sorts earlier than the e-mail we are looking for (i.e. when we
   are looking for "email" in "email>junk", a record in the mailmap
   that begins with "emaik" strictly sorts before "email", if such a
   key existed in the mailmap).

This, together with the earlier enhancement to support
case-insensitive sorting, allow us to remove an extra copy of email
buffer to downcase it.

A part of this is based on Antoine Pelisse's previous work.

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

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