Micro-optimize CanonicalTreeParser next() for ObjectWalk

ObjectWalk is invoking next() for each record we consider in a tree.
Rather than doing several method calls against the current parser,
and testing if we are at eof() at least twice per next() invocation,
do it only once and inline the logic to move the parser forward.

Change-Id: If5938f5d7b3ca24f500a184c9bd2ef193015414e
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>