pull: Clarify "helpful" message for another corner case

When the remote branch we asked for merging did not exist in the set of
fetched refs, we unconditionally hinted that it was because of lack of
configuration.  It is not necessarily so, and risks sending users for a
wild goose chase.

Make sure to check if that is indeed the case before telling a wild guess
to the user.

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -89,14 +89,15 @@ error_on_no_merge_candidates () {
 	done
 
 	curr_branch=${curr_branch#refs/heads/}
+	upstream=$(git config "branch.$curr_branch.merge")
 
 	if [ -z "$curr_branch" ]; then
 		echo "You are not currently on a branch, so I cannot use any"
 		echo "'branch.<branchname>.merge' in your configuration file."
 		echo "Please specify which branch you want to merge on the command"
 		echo "line and try again (e.g. 'git pull <repository> <refspec>')."
 		echo "See git-pull(1) for details."
-	else
+	elif [ -z "$upstream" ]; then
 		echo "You asked me to pull without telling me which branch you"
 		echo "want to merge with, and 'branch.${curr_branch}.merge' in"
 		echo "your configuration file does not tell me either.	Please"
@@ -114,6 +115,10 @@ error_on_no_merge_candidates () {
 		echo "    remote.<nickname>.fetch = <refspec>"
 		echo
 		echo "See git-config(1) for details."
+	else
+		echo "Your configuration specifies to merge the ref"
+		echo "'${upstream#refs/heads/}' from the remote, but no such ref"
+		echo "was fetched."
 	fi
 	exit 1
 }