pretty: separate out the logic to decide the use of in-body from

pretty: separate out the logic to decide the use of in-body from

When pretty-printing the log message for a given commit in the
e-mail format (e.g. "git format-patch"), we add an in-body "From:"
header when the author identity of the commit is different from the
identity of the person whose identity appears in the header of the
e-mail (the latter is passed with them "--from" option).

Split out the logic into a helper function, as we would want to
extend the condition further.

Signed-off-by: Junio C Hamano <gitster@pobox.com>