dirstate: avoid unnecessary load+dump during backup Previously, dirstate.savebackup unconditionally dumps the dirstate map to disk. It may require loading dirstate first to be able to dump it. Those operations could be expensive if the dirstate is big, and could be avoided if we know the dirstate file is up-to-date. This patch avoids the read and write if the dirstate is clean. In that case, we just do a plain copy without any serialization. This should make commands which use transactions but do not touch dirstate faster. For example, "hg bookmark -r REV NAME".