prompt: fix missing file errors in zsh

zsh seems to have a bug while redirecting the stderr of the 'read'
command:

    % read foo 2>/dev/null <foo
    zsh: no such file or directory: foo

Which causes errors to be displayed when certain files are missing.
Let's add a convenience function to manually check if the file is
readable before calling "read".

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -259,6 +259,13 @@ __git_ps1_colorize_gitstring ()
 	r="$c_clear$r"
 }
 
+eread ()
+{
+	f="$1"
+	shift
+	test -r "$f" && read "$@" <"$f"
+}
+
 # __git_ps1 accepts 0 or 1 arguments (i.e., format string)
 # when called from PS1 using command substitution
 # in this mode it prints text to add to bash PS1 prompt (includes branch name)
@@ -321,20 +328,20 @@ __git_ps1 ()
 	local step=""
 	local total=""
 	if [ -d "$g/rebase-merge" ]; then
-		read b 2>/dev/null <"$g/rebase-merge/head-name"
-		read step 2>/dev/null <"$g/rebase-merge/msgnum"
-		read total 2>/dev/null <"$g/rebase-merge/end"
+		eread "$g/rebase-merge/head-name" b
+		eread "$g/rebase-merge/msgnum" step
+		eread "$g/rebase-merge/end" total
 		if [ -f "$g/rebase-merge/interactive" ]; then
 			r="|REBASE-i"
 		else
 			r="|REBASE-m"
 		fi
 	else
 		if [ -d "$g/rebase-apply" ]; then
-			read step 2>/dev/null <"$g/rebase-apply/next"
-			read total 2>/dev/null <"$g/rebase-apply/last"
+			eread "$g/rebase-apply/next" step
+			eread "$g/rebase-apply/last" total
 			if [ -f "$g/rebase-apply/rebasing" ]; then
-				read b 2>/dev/null <"$g/rebase-apply/head-name"
+				eread "$g/rebase-apply/head-name" b
 				r="|REBASE"
 			elif [ -f "$g/rebase-apply/applying" ]; then
 				r="|AM"
@@ -358,7 +365,7 @@ __git_ps1 ()
 			b="$(git symbolic-ref HEAD 2>/dev/null)"
 		else
 			local head=""
-			if ! read head 2>/dev/null <"$g/HEAD"; then
+			if ! eread "$g/HEAD" head; then
 				if [ $pcmode = yes ]; then
 					PS1="$ps1pc_start$ps1pc_end"
 				fi