Add branch.*.merge warning and documentation update

This patch clarifies the meaning of the branch.*.merge option.
Previously, if branch.*.merge was specified but did not match any
ref, the message "No changes." was not really helpful regarding
the misconfiguration. This patch adds a warning for this.

Signed-off-by: Josef Weidendorfer <Josef.Weidendorfer@gmx.de>
Signed-off-by: Junio C Hamano <junkio@cox.net>

git-parse-remote.sh

@@ -134,6 +134,8 @@ canon_refs_list_for_fetch () {
 	# or the first one otherwise; add prefix . to the rest
 	# to prevent the secondary branches to be merged by default.
 	merge_branches=
+	found_mergeref=
+	curr_branch=
 	if test "$1" = "-d"
 	then
 		shift ; remote="$1" ; shift
@@ -171,6 +173,10 @@ canon_refs_list_for_fetch () {
 			    dot_prefix= && break
 			done
 		fi
+		if test -z $dot_prefix
+		then
+			found_mergeref=true
+		fi
 		case "$remote" in
 		'') remote=HEAD ;;
 		refs/heads/* | refs/tags/* | refs/remotes/*) ;;
@@ -191,6 +197,11 @@ canon_refs_list_for_fetch () {
 		fi
 		echo "${dot_prefix}${force}${remote}:${local}"
 	done
+	if test -z "$found_mergeref" -a "$curr_branch"
+	then
+		echo >&2 "Warning: No merge candidate found because value of config option
+         \"branch.${curr_branch}.merge\" does not match any remote branch fetched."
+	fi
 }
 
 # Returns list of src: (no store), or src:dst (store)