fixup??? built-in add -p: implement the 'g' ("goto") command

This is required by `kt/add-i-progress`.

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

Author: dscho

t/t3701-add-interactive.sh

@@ -416,13 +416,13 @@ test_expect_success 'split hunk setup' '
 test_expect_success 'goto hunk' '
 	test_when_finished "git reset" &&
 	tr _ " " >expect <<-EOF &&
-	Stage this hunk [y,n,q,a,d,K,g,/,e,?]? + 1:  -1,2 +1,3          +15
+	(2/2) Stage this hunk [y,n,q,a,d,K,g,/,e,?]? + 1:  -1,2 +1,3          +15
 	_ 2:  -2,4 +3,8          +21
 	go to which hunk? @@ -1,2 +1,3 @@
 	_10
 	+15
 	_20
-	Stage this hunk [y,n,q,a,d,j,J,g,/,e,?]?_
+	(1/2) Stage this hunk [y,n,q,a,d,j,J,g,/,e,?]?_
 	EOF
 	test_write_lines s y g 1 | git add -p >actual &&
 	tail -n 7 <actual >actual.trimmed &&