clear_commit_marks(): avoid deep recursion

clear_commit_marks(): avoid deep recursion

Before this patch, clear_commit_marks() recursed for each parent.  This
could be potentially very expensive in terms of stack space.  Probably
the only reason that this did not lead to problems is the fact that we
typically call clear_commit_marks() after marking a relatively small set
of commits.

Use (sort of) a tail recursion instead: first recurse on the parents
other than the first one, and then continue the loop with the first
parent.

Noticed by Shawn Pearce.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>