Progress report did show
 "Finding callers..."
while it should - and now does - report:
 "Searching...: x files to index (filename)"
until the index is ready to search.

Change-Id: I758baf837e19918ab24a69505a1c9bdff2efe9bb
Signed-off-by: Joerg Kubitz <jkubitz-eclipse@gmx.de>
Reviewed-on: https://git.eclipse.org/r/c/jdt/eclipse.jdt.ui/+/184486
Tested-by: JDT Bot <jdt-bot@eclipse.org>
Reviewed-by: Noopur Gupta <noopur_gupta@in.ibm.com>