fixup??? built-in add -p: implement the '/' ("search regex") command

This is necessary as long as `kt/add-i-progress` has not made it into
`maint`.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

t/t3701-add-interactive.sh

@@ -432,11 +432,11 @@ test_expect_success 'goto hunk' '
 test_expect_success 'navigate to hunk via regex' '
 	test_when_finished "git reset" &&
 	tr _ " " >expect <<-EOF &&
-	(2/2) Stage this hunk [y,n,q,a,d,K,g,/,e,?]? @@ -1,2 +1,3 @@
+	Stage this hunk [y,n,q,a,d,K,g,/,e,?]? @@ -1,2 +1,3 @@
 	_10
 	+15
 	_20
-	(1/2) Stage this hunk [y,n,q,a,d,j,J,g,/,e,?]?_
+	Stage this hunk [y,n,q,a,d,j,J,g,/,e,?]?_
 	EOF
 	test_write_lines s y /1,2 | git add -p >actual &&
 	tail -n 5 <actual >actual.trimmed &&