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.
Minimal hgk check
$ echo "[extensions]" >> $HGRCPATH
$ echo "hgk=" >> $HGRCPATH
$ hg init repo
$ cd repo
$ echo a > a
$ hg ci -Am adda
adding a
$ hg debug-cat-file commit 0
tree a0c8bcbbb45c
parent 000000000000
author test 0 0
committer test 0 0
revision 0
branch default
adda