Start with $\text{Tr}\ln(i\gamma^\mu\partial_\mu-m)$, then using the cyclic property of the trace, its linearity, and the expansion of the logarithm of an operator, you get
$$
\text{Tr}\ln(I_4\ (i\gamma^\mu\partial_\mu-m)) \\
= \text{Tr}\ln((\gamma^5)^2 \ (i\gamma^\mu\partial_\mu-m)) \\
= \text{Tr}\ln(\gamma^5(i\gamma^\mu\partial_\mu-m)\gamma^5)
$$
Then using the fact that $\{\gamma^5, \gamma^\mu\} = 0$, the $\gamma^5$'s act on the $\gamma^\mu$ by conjugation to yield $-\gamma^\mu$, while $m$ commutes with the $\gamma^5$, yielding 
$$
\text{Tr}\ln(-i\gamma^\mu\partial_\mu-m)
$$

So, using the linearity of the trace again, we can break up $\text{Tr}\ln(i\gamma^\mu\partial_\mu-m)$ into $\frac12 \text{Tr} [ \ln(i\gamma^\mu\partial_\mu-m) + \ln(i\gamma^\mu\partial_\mu-m)]$ and rewrite the second one using the previous identity.