fix-cygwin-relative-path.patch

diff --git a/src/os_unix.c b/src/os_unix.c
index 8f059be..f7f4115 100644
--- a/src/os_unix.c
+++ b/src/os_unix.c
@@ -2498,6 +2498,14 @@ mch_FullName(fname, buf, len, force)
 # else
     cygwin_conv_to_posix_path(fname, posix_fname);
 # endif
+    if (STRCMP(fname, posix_fname) != 0)
+      {
+       l = STRLEN(posix_fname);
+       while (0 < --l && posix_fname[l] == '/')
+         {
+           posix_fname[l] = NUL;
+         }
+      }
     fname = posix_fname;
 #endif

test-fnamemodify-shorten.vim

set nocompatible

function s:test()
  let r = []
  for s in [
        \ '.',
        \ './',
        \ './/',
        \ '..',
        \ '../',
        \ '..//',
        \ 'file',
        \ 'file/',
        \ 'file//',
        \ 'dir',
        \ 'dir/',
        \ 'dir//',
        \ 'notexists',
        \ 'notexists/',
        \ 'notexists//',
        \ ]
    call add(r, printf('%-16s -> %s', s, fnamemodify(s, ':.')))
  endfor
  call writefile(r, '/tmp/out.txt')
endfunction

cd /tmp
call mkdir('/tmp/dir')
call writefile([], '/tmp/file')
call s:test()

quit