parse-options: add OPT_STRING_LIST helper

This just adds repeated invocations of an option to a list
of strings. Using the "--no-<var>" form will reset the list
to empty.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>
diff --git a/t/t0040-parse-options.sh b/t/t0040-parse-options.sh
index ae26614..007f39d 100755
--- a/t/t0040-parse-options.sh
+++ b/t/t0040-parse-options.sh
@@ -28,6 +28,7 @@
     --st <st>             get another string (pervert ordering)
     -o <str>              get another string
     --default-string      set string to default
+    --list <str>          add str to list
 
 Magic arguments
     --quux                means --quux
@@ -337,4 +338,20 @@
 	test_cmp expect output
 '
 
+cat >>expect <<'EOF'
+list: foo
+list: bar
+list: baz
+EOF
+test_expect_success '--list keeps list of strings' '
+	test-parse-options --list foo --list=bar --list=baz >output &&
+	test_cmp expect output
+'
+
+test_expect_success '--no-list resets list' '
+	test-parse-options --list=other --list=irrelevant --list=options \
+		--no-list --list=foo --list=bar --list=baz >output &&
+	test_cmp expect output
+'
+
 test_done