git-difftool: allow skipping file by typing 'n' at prompt

This is useful if you forgot to restrict the diff to the paths you want
to see, or selecting precisely the ones you want is too much typing.

[jc: with a change to return from the function upon 'n' by Charles Bailey
and a small tweak in stdin_doesnot_contain() in the test]

Signed-off-by: Sitaram Chamarty <sitaram@atc.tcs.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-difftool--helper.sh

@@ -43,12 +43,15 @@ launch_merge_tool () {
 		printf "\nViewing: '$MERGED'\n"
 		if use_ext_cmd
 		then
-			printf "Hit return to launch '%s': " \
+			printf "Launch '%s' [Y/n]: " \
 				"$GIT_DIFFTOOL_EXTCMD"
 		else
-			printf "Hit return to launch '%s': " "$merge_tool"
+			printf "Launch '%s' [Y/n]: " "$merge_tool"
+		fi
+		if read ans && test "$ans" = n
+		then
+			return
 		fi
-		read ans
 	fi
 
 	if use_ext_cmd

t/t7800-difftool.sh

@@ -38,7 +38,17 @@ restore_test_defaults()
 prompt_given()
 {
 	prompt="$1"
-	test "$prompt" = "Hit return to launch 'test-tool': branch"
+	test "$prompt" = "Launch 'test-tool' [Y/n]: branch"
+}
+
+stdin_contains()
+{
+	grep >/dev/null "$1"
+}
+
+stdin_doesnot_contain()
+{
+	! stdin_contains "$1"
 }
 
 # Create a file on master and change it on branch
@@ -265,4 +275,35 @@ test_expect_success PERL 'difftool --extcmd cat arg2' '
 	test "$diff" = branch
 '
 
+# Create a second file on master and a different version on branch
+test_expect_success PERL 'setup with 2 files different' '
+	echo m2 >file2 &&
+	git add file2 &&
+	git commit -m "added file2" &&
+
+	git checkout branch &&
+	echo br2 >file2 &&
+	git add file2 &&
+	git commit -a -m "branch changed file2" &&
+	git checkout master
+'
+
+test_expect_success PERL 'say no to the first file' '
+	diff=$((echo n; echo) | git difftool -x cat branch) &&
+
+	echo "$diff" | stdin_contains m2 &&
+	echo "$diff" | stdin_contains br2 &&
+	echo "$diff" | stdin_doesnot_contain master &&
+	echo "$diff" | stdin_doesnot_contain branch
+'
+
+test_expect_success PERL 'say no to the second file' '
+	diff=$((echo; echo n) | git difftool -x cat branch) &&
+
+	echo "$diff" | stdin_contains master &&
+	echo "$diff" | stdin_contains branch &&
+	echo "$diff" | stdin_doesnot_contain m2 &&
+	echo "$diff" | stdin_doesnot_contain br2
+'
+
 test_done