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.
Test update logic when there are renames
Update with local changes across a file rename
$ hg init
$ echo a > a
$ hg add a
$ hg ci -m a
$ hg mv a b
$ hg ci -m rename
$ echo b > b
$ hg ci -m change
$ hg up -q 0
$ echo c > a
$ hg up
merging a and b to b
warning: conflicts during merge.
merging b incomplete! (edit conflicts, then use 'hg resolve --mark')
0 files updated, 0 files merged, 0 files removed, 1 files unresolved
use 'hg resolve' to retry unresolved file merges
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