taskd/clone.sh: do not rely on SIGXFSZ error
The actual git process that ends up dying as a result of a SIGXFSZ
when it attempts to create a file that's too large will almost
certainly not be the initial git process that's running the fetch.
As a result, the actual error exit code will end up being something
much less helpful.
Attempt to detect this situation by looking for any files that
have the same size as the file size limit and trigger the exceeds
file size limit that way as well.
Signed-off-by: Kyle J. McKay <mackyle@gmail.com>