bug#43250: 28.0.50; tramp ignores .ssh/config

[Sam Steingold <sds@gnu.org>]

I have ~/.ssh/config that says

--8<---------------cut here---------------start------------->8---
Host            fence
HostName        fencepost.gnu.org

Host            *
Compression     yes
ForwardAgent    yes
ForwardX11      no
User            sds
VisualHostKey   no
GSSAPIDelegateCredentials yes
--8<---------------cut here---------------end--------------->8---

so I can do `ssh fence` from shell just fine.

Until recently I could also do

--8<---------------cut here---------------start------------->8---
(copy-file "/scp:fence:~/.newsrc" "~/.newsrc")
--8<---------------cut here---------------end--------------->8---

using tramp, but now I am getting this weird question

--8<---------------cut here---------------start------------->8---
scp user name for fence (default sam.steingold): 
--8<---------------cut here---------------end--------------->8---

with this stack:

--8<---------------cut here---------------start------------->8---
Debugger entered--Lisp error: (quit "")
  signal(quit (""))
  tramp-signal-hook-function(quit (""))
  signal(quit (""))
  tramp-maybe-open-connection((tramp-file-name "scp" nil nil "fence" nil "~/.newsrc" nil))
  tramp-send-command((tramp-file-name "scp" nil nil "fence" nil "~/.newsrc" nil) "cd ~ && pwd")
  tramp-sh-handle-expand-file-name("/scp:fence:~/.newsrc" nil)
  apply(tramp-sh-handle-expand-file-name ("/scp:fence:~/.newsrc" nil))
  tramp-sh-file-name-handler(expand-file-name "/scp:fence:~/.newsrc" nil)
  apply(tramp-sh-file-name-handler expand-file-name ("/scp:fence:~/.newsrc" nil))
  tramp-file-name-handler(expand-file-name "/scp:fence:~/.newsrc" nil)
  file-symlink-p("/scp:fence:~/.newsrc")
  file-chase-links("/scp:fence:~/.newsrc")
...
--8<---------------cut here---------------end--------------->8---

This looks like a newly introduced bug.
Thank you.


In GNU Emacs 28.0.50 (build 8, x86_64-apple-darwin19.6.0, NS appkit-1894.60 Version 10.15.6 (Build 19G2021))
 of 2020-08-31 built on BZ-C02XR5CGJG5L
Repository revision: 04578c10636fbbd1c1a924404a955eb37ffefd8f
Repository branch: master
Windowing system distributor 'Apple', version 10.3.1894
System Description:  Mac OS X 10.15.6

Configured using:
 'configure --with-imagemagick --with-mailutils --with-ns
 PKG_CONFIG_PATH=/usr/local/opt/libxml2/lib/pkgconfig:/usr/local/opt/imagemagick/lib/pkgconfig:/usr/local/opt/gnutls/lib/pkgconfig:/usr/local/opt/jansson/lib/pkgconfig:/usr/local/opt/libtiff/lib/pkgconfig:/usr/local/opt/libpng/lib/pkgconfig:/usr/local/opt/libjpeg/lib/pkgconfig:/usr/local/opt/freetype/lib/pkgconfig'

Configured features:
JPEG TIFF PNG IMAGEMAGICK NOTIFY KQUEUE ACL GNUTLS LIBXML2 ZLIB
TOOLKIT_SCROLL_BARS NS MODULES THREADS JSON PDUMPER LCMS2

Important settings:
  value of $LC_ALL: en_US.UTF-8
  value of $LANG: C
  locale-coding-system: utf-8-unix

Major mode: Debugger

Minor modes in effect:
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  shell-dirtrack-mode: t
  global-edit-server-edit-mode: t
  winner-mode: t
  which-function-mode: t
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  tooltip-mode: t
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  mouse-wheel-mode: t
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  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  column-number-mode: t
  line-number-mode: t

Load-path shadows:
None found.

Features:
(shadow sort bbdb-message mailalias cookie1 mail-extr gnus-msg gnus-art
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Memory information:
((conses 16 385411 16621)
 (symbols 48 27965 4)
 (strings 32 208578 4330)
 (string-bytes 1 5400862)
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-- 
Sam Steingold (http://sds.podval.org/) on darwin Ns 10.3.1894
http://childpsy.net http://calmchildstories.com http://steingoldpsychology.com
http://think-israel.org https://www.peaceandtolerance.org/
Incorrect time synchronization.