mail.sh: include copy/rename detection warnings

mail.sh: include copy/rename detection warnings

When performing diffs via diff-tree, mail.sh attempts to show renames
and copies if there are any.

For non-exact copies and renames, this detection can be expensive.
Git limits its efforts in this regard when there are many possibilities
to test (see the `diff.renameLimit` config item).

If the number of items to check exceeds the limit, Git spouts
warnings to STDERR about it including, if appropriate, a possible
new setting for the `diff.renameLimit` config.

Previously these warnings would just clutter up taskd.pl's STDERR
output.

Instead, include the warnings in the diffs that get sent out as
they are relevant to readers of the diffs as to whether or not there
might be undetected non-exact copies and/or renames present.

But the value to increase `diff.renameLimit` to is not relevant to
the diffs included in mail.sh's notifications, therefore suppress
any of those that show up while including the relevant warnings.

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