fast-export: differentiate between explicitly UTF-8 and implicitly UTF-8
The find_encoding() function returned the encoding used by a commit
message, returning a default of git_commit_encoding (usually UTF-8).
Although the current code does not differentiate between a commit which
explicitly requested UTF-8 and one where we just assume UTF-8 because no
encoding is set, it will become important when we try to preserve the
encoding header. Since is_encoding_utf8() returns true when passed
NULL, we can just return NULL from find_encoding() instead of returning
git_commit_encoding.
Signed-off-by: Elijah Newren <newren@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>