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>

Documentation/config.txt

@@ -125,10 +125,17 @@ apply.whitespace::
 
 branch.<name>.remote::
 	When in branch <name>, it tells `git fetch` which remote to fetch.
+	If this option is not given, `git fetch` defaults to remote "origin".
 
 branch.<name>.merge::
-	When in branch <name>, it tells `git fetch` the default remote branch
-	to be merged.
+	When in branch <name>, it tells `git fetch` the default refspec to
+	be marked for merging in FETCH_HEAD. The value has exactly to match
+	a remote part of one of the refspecs which are fetched from the remote
+	given by "branch.<name>.remote".
+	The merge information is used by `git pull` (which at first calls
+	`git fetch`) to lookup the default branch for merging. Without
+	this option, `git pull` defaults to merge the first refspec fetched.
+	Specify multiple values to get an octopus merge.
 
 pager.color::
 	A boolean to enable/disable colored output when the pager is in

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)