git-pull.sh: better warning message for "git pull" on detached head.

Otherwise, git complains about not finding a branch to pull from in
'branch..merge', which is hardly understandable. While we're there,
reword the sentences slightly.

Signed-off-by: Matthieu Moy <Matthieu.Moy@imag.fr>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -90,12 +90,19 @@ error_on_no_merge_candidates () {
 
 	curr_branch=${curr_branch#refs/heads/}
 
+	if [ -z "$curr_branch" ]; then
+		echo "You are not currently on a branch, so I cannot use any"
+		echo "'branch.<branchname>.merge' in your configuration file."
+		echo "Please specify which branch you want to merge on the command"
+		echo "line and try again (e.g. 'git pull <repository> <refspec>')."
+		echo "See git-pull(1) for details."
+	else
 		echo "You asked me to pull without telling me which branch you"
 		echo "want to merge with, and 'branch.${curr_branch}.merge' in"
 		echo "your configuration file does not tell me either.	Please"
-	echo "name which branch you want to merge on the command line and"
+		echo "specify which branch you want to merge on the command line and"
 		echo "try again (e.g. 'git pull <repository> <refspec>')."
-	echo "See git-pull(1) for details on the refspec."
+		echo "See git-pull(1) for details."
 		echo
 		echo "If you often merge with the same branch, you may want to"
 		echo "configure the following variables in your configuration"
@@ -107,6 +114,7 @@ error_on_no_merge_candidates () {
 		echo "    remote.<nickname>.fetch = <refspec>"
 		echo
 		echo "See git-config(1) for details."
+	fi
 	exit 1
 }