rev-list: move `show_commit()` to the bottom

rev-list: move `show_commit()` to the bottom

The `show_commit()` function already depends on `finish_commit()`, and
in the upcoming commit, we'll also add a dependency on
`finish_object__ma()`. Since in C symbols must be declared before
they're used, let's move `show_commit()` below both `finish_commit()`
and `finish_object__ma()`, so the code is cleaner as a whole without the
need for declarations.

Signed-off-by: Karthik Nayak <karthik.188@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>