tags-loop-revert-buffers

tags-loop-revert-buffers is a variable defined in `etags.el'.
Its value is

nil

Documentation:
Non-nil means tags-scanning loops should offer to reread changed files.
These loops normally read each file into Emacs, but when a file
is already visited, they use the existing buffer.
When this flag is non-nil, they offer to revert the existing buffer
in the case where the file has changed since you visited it.

You can customize this variable.