bug#26156: 26.0.50; expand-file-name fails to expand ~ if tramp is loaded and "/" is specified

[mano155]

On the most recent version, if tramp is loaded, expand-file-name does
not expand ~ properly when the second argument is "/". This breaks
breaks magit-clone, though weirdly, that command only fails if the
active point is in a dired buffer. To clarify, "~" is absolute (per the
docstring for file-name-absolute-p), but it is not canonical (per the
docstring for expand-file-name).

Bisecting, the breaking change was 19a04b4c32. 

Starting from "emacs -Q":

(expand-file-name "~" "/")
"/home/sh0e"

(expand-file-name "~/" "/")
"/home/sh0e/"

(require 'tramp)
tramp

(expand-file-name "~" "/")
"~"

(expand-file-name "~/" "/")
"~/build/"


In GNU Emacs 26.0.50 (build 2, x86_64-unknown-linux-gnu, GTK+ Version 3.22.10)
 of 2017-03-14 built on topd0g
Repository revision: c66aaa61639e72a70a4f2c4bc73645048caebe53
Windowing system distributor 'The X.Org Foundation', version 11.0.11902000
System Description:     Arch Linux

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Configured using:
 'configure --with-xwidgets'

Configured features:
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NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
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Important settings:
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  value of $XMODIFIERS: @im=fcitx
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Major mode: Lisp Interaction

Minor modes in effect:
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  auto-composition-mode: t
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Load-path shadows:
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Features:
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Memory information:
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 (symbols 48 23632 17)
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 (strings 32 27983 6861)
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