bug#27315: Regression: `abbreviate-file-name' and `tramp-file-name-handler'

[Alexander Shukaev <emacs@Alexander.Shukaev.name>]

Hello,

In Emacs 26 series, I notice regression that has to do with 
`abbreviate-file-name' and `tramp-file-name-handler'.  Here is how to 
reproduce it:

$ emacs -Q

Paste the following to the "*scratch*" buffer:

(setq-default
   frame-title-format
   '((:eval (list (user-login-name)
                  "@"
                  (system-name)
                  ":"
                  (abbreviate-file-name (or (buffer-file-name)
                                            (file-name-as-directory
                                             default-directory)))))))

M-x eval-buffer
M-x find-file /sudo:root@localhost:~/hang

will result in hanging with the "Sending password" message in 
minibuffer.  I did not debug this, but I know for sure how to workaround 
this and, therefore, can provide a clue how to fix it.  That is

$ emacs -Q

Paste the following to the "*scratch*" buffer:

(setq-default
   frame-title-format
   '((:eval (list (user-login-name)
                  "@"
                  (system-name)
                  ":"
                  (let (file-name-handler-alist)
                    (abbreviate-file-name (or (buffer-file-name)
                                              (file-name-as-directory
                                               default-directory))))))))

M-x eval-buffer
M-x find-file /sudo:root@localhost:~/yuppie

opens the file just fine.  It means that somehow since Emacs 26 
`abbreviate-file-name' either triggers `tramp-file-name-handler' or it 
did so before but this time (I heard that the TRAMP syntax has changed) 
it might be bugged.

Michael, could you have a look at this?  Thanks!

Kind regards,
Alexander