shared/edit-util: split out function to populate temp file

shared/edit-util: split out function to populate temp file

In preparation for future changes: I want to add a mode where interactive
editing is not done, and when this preparation is moved to a helper, it's much
easier to skip it.

e->line is initialized to 1 and overwritten even if sync fails. Theoretically
this is against our style, but the alternative is to propagate a temporary
value of line through the layers, which adds a lot of noise. If we fail, this
EditFile object will not be used for anything, to the changed value of .line
has no effect.