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
lib - sys sys
mercurial - sys sys
hgrc.d - sys sys
9diff.rc - sys sys
factotum.rc - sys sys
rc - sys sys
bin - sys sys
9diff - sys sys
hg - sys sys
sys - sys sys
lib - sys sys
python - sys sys
lib - sys sys
python2.5 - sys sys
site-packages - sys sys
hgext - sys sys
+ - sys sys
mercurial - sys sys
+ - sys sys
mercurial-VERSION-py2.5.egg-info - sys sys
src - sys sys
cmd - sys sys
hg - sys sys
+ - sys sys