store: optimze _auxencode() a bit by grouping the reserved names by length
This reduces perffncacheencode wall time on Windows 7 x64 for my netbeans clone
here from 4.3 to 4.0 (7% faster).
$ 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