git-pull: refuse default merge without branch.*.merge

Everybody hated the pull behaviour of merging the first branch
listed on remotes/* file (or remote.*.fetch config) into the
current branch.  This finally corrects that UI wart by
forbidding "git pull" without an explicit branch name on the
command line or branch.$current.merge for the current branch.

The matching change to git-clone was made to prepare the default
branch.*.merge entry for the primary branch some time ago.

Signed-off-by: Junio C Hamano <junkio@cox.net>

git-parse-remote.sh

@@ -144,7 +144,8 @@ canon_refs_list_for_fetch () {
 			curr_branch=$(git-symbolic-ref HEAD | \
 			    sed -e 's|^refs/heads/||')
 			merge_branches=$(git-repo-config \
-			    --get-all "branch.${curr_branch}.merge")
+			    --get-all "branch.${curr_branch}.merge") ||
+			merge_branches=.this.would.never.match.any.ref.
 		fi
 		set x $(expand_refs_wildcard "$@")
 		shift