dirs.addpath: rework algorithm to search forward
This improves performance because it uses strchr rather than a loop.
For LLVM/clang version "Apple LLVM version 6.0 (clang-600.0.56) (based on LLVM
3.5svn)" on OS X, for a repo with over 200,000 files, this improves perfdirs
from 0.248 seconds to 0.230 (7.3%)
For gcc 4.4.6 on Linux, for a test repo with over 500,000 files, this improves
perfdirs from 0.704 seconds to 0.658 (6.5%).
$ hg init
$ echo This is file a1 > a
$ hg add a
$ hg commit -m "commit #0"
$ ls
a
$ echo This is file b1 > b
$ hg add b
$ hg commit -m "commit #1"
$ hg co 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
B should disappear
$ ls
a