repo bash completion: _find_repo: improve check

Checking if main.py exists is better than checking if it
is a file, since it might be a symlink.

Change-Id: I1a9e617470a05c2161479cd3f0a10712a2b9d2c2

bash_completion/repo.bash

@@ -46,7 +46,7 @@ _find_repo() {
 
     while [ "${dir}" != / ]
     do
-        if [ -f "${dir}/.repo/repo/main.py" ]
+        if [ -e "${dir}/.repo/repo/main.py" ]
         then
             found=0
             break