mail.sh: always --find-copies-harder

mail.sh: always --find-copies-harder

Except when showing a one-line summary without any diff, mail.sh
used "-B --find-copies-harder" on all the other diff operations
except for one.

Make it consistent and always use "-B --find-copies-harder" so
there's no confusion between the individual diffs and the summary
diff shown at the end.

Without --find-copies-harder, Git will simply not show even 100%
exact copies as copies if the original file the copy came from
was not also modified in the same commit as the copy.

With the option Git will ALWAYS find exact copies regardless of
how many candidates there are or what the value of `diff.renameLimit`
has been set to.

Signed-off-by: Kyle J. McKay <mackyle@gmail.com>