builtin-mailinfo.c: compare character encodings case insensitively

builtin-mailinfo.c: compare character encodings case insensitively

When converting between character encodings, git tests whether the "from"
encoding and the "to" encoding have the same name.  git should perform this
test case insensitively so that e.g. utf-8 is not seen as a different
encoding than UTF-8.

Additionally, it is not necessary to call tolower() anymore on the encodings
extracted from the mail message.

Signed-off-by: Brandon Casey <casey@nrlssc.navy.mil>
Signed-off-by: Junio C Hamano <gitster@pobox.com>