mail.sh: tolerate running in a linked working tree
The mail.sh script will not normally ever be run from within
a linked working tree.
However, such a feature could be useful for testing or perhaps
some special kind of customization.
Therefore support running from a current directory that is
located within a linked working tree.
Practically the only effect of this change is to show the
correct project description when run from a linked working
tree rather than "UNNAMED PROJECT" as nothing else is
sensitive to running from a linked working tree.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>