chg: let procutil maintain its own pagerpid
Previously, chg.c maintains the pagerpid. Let's move it to procutil.c.
Note: chg.c still have a pagerpid to decide whether to call attachio or not.
In the future, attachio may be moved from hgc_open to hgc_runcommand, and
hgc_runcommand handles both pager and attachio so we don't need to run
attachio twice. And chg.c will be free of pagerpid.
$ rm -rf a
$ hg init a
$ cd a
$ echo a > a
$ hg ci -Am0
adding a
$ hg tag t1 # 1
$ hg tag --remove t1 # 2
$ hg co 1
1 files updated, 0 files merged, 0 files removed, 0 files unresolved
$ hg tag -f -r0 t1
$ hg tags
tip 3:a49829c4fc11
t1 0:f7b1eb17ad24
$ cd ..