". . . the staff of army life was called "hartack."
Clearly, this is a minor corruption of “hard tack,” the VERY hard and frequently weevily ration biscuit – really, more like a thick, unsalted cracker. It was standard fare aboard Royal Navy vessels in the early 1800s, as referenced in the writings of C. S. Forester (the Horatio Hornblower series.)
“Grab a root” is likely a shortening of the phrase, “Grab a root and growl.” I often heard in my mother's family, which came to East Texas from the Carolinas both before and after the War of Northern Aggression. It apparently originally referred to field bowel evacuation, with no privy handy. It came to mean that one should “buckle down” and complete an unpleasant but necessary task.
I never even thought of “Hunt your holes” as a particularly quaint turn of phrase. It is simply a heads up, warning that “We're about come under fire; find cover.” In the past couple of decades, I've heard it most often in the context of local politics.