phases: make advance/retractboundary() atomic Before this, if advanceboundary() failed after updating some roots but before calling retractboundary(), the phase cache would be left in an invalid state, marked dirty, and written as such. This patch approach is to turn advance/retractboundary() into phasecache methods, then operate on copies and merge them back on success. With the same technique, we can ensure the atomicity of combinations of advance/retractboundary() calls, like those performed in changegroup handling code.