windows: use 'str.replace()' instead of combination of split() and join()
'str.replace()' is twice (or more) as fast as combination of split()
and join().
combination of split() and join() in current 'util.pconvert()'
implementation comes only from historical reason when win32mbcs
extension was introduced.
http://mercurial.selenic.com/bts/issue1438
$ "$TESTDIR/hghave" symlink || exit 80
$ hg init
$ ln -s foo link
$ hg add link
$ hg ci -mbad link
$ hg rm link
$ hg ci -mok
$ hg diff -g -r 0:1 > bad.patch
$ hg up 0
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg import --no-commit bad.patch
applying bad.patch
$ hg status
R link
? bad.patch