dirs: use mutable strings internally...
dirs: use mutable strings internally
perfdirs results for a working dir with 170,000 files:
Python 638 msec
C 244
C+int 192
C+int+str 168
In the large repo above, the nearly 0.5 second time improvement is
visible in commands like "hg add" and "hg update".
hg add
Python 1100 msec
C+int+str 600
hg update (with nothing to do)
Python 2800 msec
C+int+str 2240
