wait-on-file: properly wait on any files and symlink

This make the utility more useful, for example to wait on a lock file.
We also add an explicit -L check since the lock are "weird" symlink.

Differential Revision: https://phab.mercurial-scm.org/D12617

tests/testlib/wait-on-file

@@ -29,7 +29,7 b' if [ -n "$create" ]; then'
     touch "$create"
     create=""
 fi
-while [ "$timer" -gt 0 ] && [ ! -f "$wait_on" ]; do
+while [ "$timer" -gt 0 ] && !([ -e "$wait_on" ] || [ -L "$wait_on" ]) ; do
     timer=$(( $timer - 1))
     sleep 0.02
 done