cat-file: add mailmap support to -s option

cat-file: add mailmap support to -s option

Even though the cat-file command with `-s` option does not complain when
`--use-mailmap` option is given, the latter option is ignored. Compute
the size of the object after replacing the idents and report it instead.

In order to make `-s` option honour the mailmap mechanism we have to
read the contents of the commit/tag object. Make use of the call to
`oid_object_info_extended()` to get the contents of the object and store
in `buf`. `buf` is later freed in the function.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: John Cai <johncai86@gmail.com>
Helped-by: Taylor Blau <me@ttaylorr.com>
Helped-by: Ævar Arnfjörð Bjarmason <avarab@gmail.com>
Signed-off-by: Siddharth Asthana <siddharthasthana31@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>