mailinfo: load default metainfo_charset lazily

mailinfo: load default metainfo_charset lazily

In a later change, we will use parse_option to parse mailinfo's options.
In mailinfo, both "-u", "-n", and "--encoding" try to set the same
field, with "-u" reset that field to some default value from
configuration variable "i18n.commitEncoding".

Let's delay the setting of that field until we finish processing all
options. By doing that, "i18n.commitEncoding" can be parsed on demand.
More importantly, it cleans the way for using parse_option.

This change introduces some inconsistent brackets "{}" in "if/else if"
construct, however, we will rewrite them in the next few changes.

Signed-off-by: Đoàn Trần Công Danh <congdanhqx@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>