difftool--helper: exit when reading a prompt answer fails

[Johannes Sixt <j6t@kdbg.org>]

An attempt to quit difftool by hitting Ctrl-D (EOF) at its prompt does
not quit it, but is treated as if 'yes' was answered to the prompt and
all following prompts, which is contrary to the user's intent. Fix the
error check.

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
---
 Found while reviewing your latest patch.

 I chose the 'foo || return' idiom for the error check, but left the
 'if' for the interesting check, because I feel it is more readable
 than 'test ... && return'.

 -- Hannes

 git-difftool--helper.sh | 3 ++-
 t/t7800-difftool.sh     | 8 ++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/git-difftool--helper.sh b/git-difftool--helper.sh
index 7ef36b9..aca0413 100755
--- a/git-difftool--helper.sh
+++ b/git-difftool--helper.sh
@@ -49,7 +49,8 @@ launch_merge_tool () {
 		else
 			printf "Launch '%s' [Y/n]: " "$merge_tool"
 		fi
-		if read ans && test "$ans" = n
+		read ans || return
+		if test "$ans" = n
 		then
 			return
 		fi
diff --git a/t/t7800-difftool.sh b/t/t7800-difftool.sh
index dc30a51..9cf5dc9 100755
--- a/t/t7800-difftool.sh
+++ b/t/t7800-difftool.sh
@@ -301,6 +301,14 @@ test_expect_success PERL 'say no to the second file' '
 	! grep br2 output
 '
 
+test_expect_success PERL 'ending prompt input with EOF' '
+	git difftool -x cat branch </dev/null >output &&
+	! grep master output &&
+	! grep branch output &&
+	! grep m2 output &&
+	! grep br2 output
+'
+
 test_expect_success PERL 'difftool --tool-help' '
 	git difftool --tool-help >output &&
 	grep tool output
-- 
2.0.0.12.gbcf935e