runtime(ftplugin): allow to exec if curdir is in PATH

In case the current directory is present as valid $PATH entry, it is OK
to call the program from it, even if vim curdir is in that same
directory.

(Without that patch, for instance, you will not be able to open .zip
files while your current directory is /bin)

closes: #13027

Signed-off-by: Christian Brabandt <cb@256bit.org>
diff --git a/runtime/ftplugin/ruby.vim b/runtime/ftplugin/ruby.vim
index daffe1e..a424801 100644
--- a/runtime/ftplugin/ruby.vim
+++ b/runtime/ftplugin/ruby.vim
@@ -77,11 +77,14 @@
   let cwd = fnameescape(getcwd())
   try
     exe cd fnameescape(a:root)
-    if fnamemodify(exepath('ruby'), ':p:h') ==# cwd
+    let s:tmp_cwd = getcwd()
+    if (fnamemodify(exepath('ruby'), ':p:h') ==# cwd
+          \ && (index(split($PATH,has("win32")? ';' : ':'), s:tmp_cwd) == -1 || s:tmp_cwd == '.'))
       let path = []
     else
       let path = split(system(path_check),',')
     endif
+    unlet s:tmp_cwd
     exe cd cwd
     return path
   finally