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".

mercurial/dirstate.py

@@ -1220,6 +1220,7 b' class dirstate(object):'
         # use '_writedirstate' instead of 'write' to write changes certainly,
         # because the latter omits writing out if transaction is running.
         # output file will be used to create backup of dirstate at this point.
+        if self._dirty or not self._opener.exists(filename):
         self._writedirstate(self._opener(filename, "w", atomictemp=True,
                                          checkambig=True))