agnoster: fix git working tree check (#9072)

`git rev-parse --is-inside-work-tree` does not have a different exit
code when not within a work tree; the difference is in the output,
either "true" when in a work tree, or "false" when not.

The if was modified similarly to the check for the hide-status config,
which also parses command output rather than exit code.

Co-authored-by: Kyle Laker <klaker@easydynamics.com>