The “//” fix confuses cygwin-mount

[Chris Zheng]

Dear List,

The commit c24c5dc4a4cc18e7f1ec949efcfe1d4bae541d02 confuses the
cygwin-mount that serves the translation from “/c/Windows” to
“C:/Windows/” under MS-Windows. With the above commit and
cygwin-mount,

(substitute-in-file-name "/method:host:/foo")
⇒ "/method:host:c:/msys64/foo"

While before that commit the result is "/method:host:/foo".

The reason is that in ‘tramp-handle-substitute-in-file-name’ from
line 3563

(setq filename
      (concat (file-remote-p filename)
              (substitute-in-file-name localname))))))
              ^ (substitute-in-file-name "/foo") results in "c:/msys64/foo".

Previously the whole "/method:host:/foo" was translated by the
‘substitute-in-file-name’ thus no problem occurs.

Could you please have a look at this.

Thank you very much,

Chris