cmdutil: separate building commit text from 'commitforceeditor'
This separation makes it easier to extend/hook building commit text
from the specified context.
This patch uses 'committext' instead of 'edittext' for names of newly
added variable and function, because the former is more purpose
specific than the latter, even though 'edittext' in 'buildcommittext'
is left as it is to reduce amount of diff.
$ 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 ..