[O] Asks user what to do with each tangle-file before overriding?

[Chunyang Xu]

Hi,

Currently 'C-c C-v C-t' ('org-babel-tangle') simply overrides existing
tangle-file, I would like org to ask me what to do? such as

a) yes (override)
b) no (don't override)
c) show the diff then ask again

In addition, if there is no diff (i.e., having the same contents), user
can choose (e.g., via a user option) to simply pass this tangle-file
without overriding.

I am asking for this feature because I am storing my dotfiles in a
single Org file [1], and when I change my Bash configuration then
tangle, I don't need to worry if Org is tangling other configuration
correctly, especially ~/.ssh/id_rsa and ~/.authinfo.gpg. I would rather
leave these unchanged files than override (even only file modification
timestamps are updated).

[1] https://raw.githubusercontent.com/xuchunyang/dotfiles/master/README.org