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 '.'.
http://mercurial.selenic.com/bts/issue619
$ hg init
$ echo a > a
$ hg ci -Ama
adding a
$ echo b > b
$ hg branch b
marked working directory as branch b
(branches are permanent and global, did you want a bookmark?)
$ hg ci -Amb
adding b
$ hg co -C 0
0 files updated, 0 files merged, 1 files removed, 0 files unresolved
Fast-forward:
$ hg merge b
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
(branch merge, don't forget to commit)
$ hg ci -Ammerge
Bogus fast-forward should fail:
$ hg merge b
abort: merging with a working directory ancestor has no effect
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