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>

builtin/fast-export.c

@@ -453,7 +453,7 @@ static const char *find_encoding(const char *begin, const char *end)
 	bol = memmem(begin, end ? end - begin : strlen(begin),
 		     needle, strlen(needle));
 	if (!bol)
-		return git_commit_encoding;
+		return NULL;
 	bol += strlen(needle);
 	eol = strchrnul(bol, '\n');
 	*eol = '\0';