dirstate: prevent useless util.fspath() invocation for '.'
at first of dirstate.walk() on case insensitive filesystem,
normalization of '.' causes util.fspath() invocation, but '.' is not
cached in it.
this invocation is not only useless, but also harmful: initial "hg
tag" causes creation of ".hgtags" file after dirstate.walk(), and
looking up ".hgtags" in cache will fail, because directory contents of
root is already cached at util.fspath() invocation for '.'.
Just exercize debugindexdot
Create a short file history including a merge.
$ hg init t
$ cd t
$ echo a > a
$ hg ci -qAm t1 -d '0 0'
$ echo a >> a
$ hg ci -m t2 -d '1 0'
$ hg up -qC 0
$ echo b >> a
$ hg ci -m t3 -d '2 0'
created new head
$ HGMERGE=true hg merge -q
$ hg ci -m merge -d '3 0'
$ hg debugindexdot .hg/store/data/a.i
digraph G {
-1 -> 0
0 -> 1
0 -> 2
2 -> 3
1 -> 3
}