MyFirstObjectWalk: remove unnecessary conditional statement

MyFirstObjectWalk: remove unnecessary conditional statement

In the given example, `commit` cannot be `NULL` (because this is the
loop condition: if it was `NULL`, the loop body would not be entered at
all). It took this developer a moment or two to see that this is
therefore dead code.

Let's remove it, to avoid puzzling future readers.

Signed-off-by: Johannes Schindelin <redacted>
Reviewed-by: Emily Shaffer <redacted>
Signed-off-by: Junio C Hamano <redacted>