wait-on-file: use proper variable in math

This seems better and safer to be explicit.

Differential Revision: https://phab.mercurial-scm.org/D8426

tests/testlib/wait-on-file

@@ -1,38 +1,38 b''
 #!/bin/bash
 #
 # wait up to TIMEOUT seconds until a WAIT_ON_FILE is created.
 #
 # In addition, this script can create CREATE_FILE once it is ready to wait.
 
 if [ $# -lt 2 ] || [ $# -gt 3 ]; then
     echo $#
     echo "USAGE: $0 TIMEOUT WAIT_ON_FILE [CREATE_FILE]"
 fi
 
 timer="$1"
 
 # if the test timeout have been extended, explicitly extend the provided timer
 if [ "$HGTEST_TIMEOUT_DEFAULT" -lt "$HGTEST_TIMEOUT" ]; then
     timer=$(( ($timer * $HGTEST_TIMEOUT) / $HGTEST_TIMEOUT_DEFAULT ))
 fi
 
 wait_on="$2"
 create=""
 if [ $# -eq 3 ]; then
     create="$3"
 fi
 
 if [ -n "$create" ];
 then
     touch "$create"
     create=""
 fi
 while [ "$timer" -gt 0 ] && [ ! -f "$wait_on" ];
 do
-    timer=$(( timer - 1))
+    timer=$(( $timer - 1))
     sleep 0.01
 done
 if [ "$timer" -le 0 ]; then
     echo "file not created after $1 seconds: $wait_on" >&2
     exit 1
 fi