Pull request: all: fix client custom upstream comments

Updates #2947.

Squashed commit of the following:

commit 498a05459b1aa00bcffee490acfeecb567025971
Merge: 6a7a2f87 21e2c419
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 13 13:40:29 2021 +0300

    Merge branch 'master' into 2947-cli-ups-comment

commit 6a7a2f87cfd2bdd829b82889890511fef8d84b9b
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 13 13:34:28 2021 +0300

    all: imp code, tests

commit abc0be239f69cfc3e7d0cde2fc952d9157b2cd5d
Merge: 82fb3fcb 6410feeb
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Tue Apr 13 13:17:09 2021 +0300

    Merge branch 'master' into 2947-cli-ups-comment

commit 82fb3fcb49cbc8d439cb5959c1cb84ae49b2257e
Author: Ainar Garipov <A.Garipov@AdGuard.COM>
Date:   Mon Apr 12 22:13:41 2021 +0300

    all: fix client custom upstream comments

internal/aghstrings/strings.go

@@ -19,6 +19,18 @@ func CloneSlice(a []string) (b []string) {
 	return CloneSliceOrEmpty(a)
 }
 
+// FilterOut returns a copy of strs with all strings for which f returned true
+// removed.
+func FilterOut(strs []string, f func(s string) (ok bool)) (filtered []string) {
+	for _, s := range strs {
+		if !f(s) {
+			filtered = append(filtered, s)
+		}
+	}
+
+	return filtered
+}
+
 // InSlice checks if string is in the slice of strings.
 func InSlice(strs []string, str string) (ok bool) {
 	for _, s := range strs {
@@ -30,6 +42,12 @@ func InSlice(strs []string, str string) (ok bool) {
 	return false
 }
 
+// IsCommentOrEmpty returns true of the string starts with a "#" character or is
+// an empty string.
+func IsCommentOrEmpty(s string) (ok bool) {
+	return len(s) == 0 || s[0] == '#'
+}
+
 // SplitNext splits string by a byte and returns the first chunk skipping empty
 // ones.  Whitespaces are trimmed.
 func SplitNext(s *string, sep rune) (chunk string) {

internal/aghstrings/strings_test.go

@@ -36,6 +36,21 @@ func TestCloneSlice_family(t *testing.T) {
 	})
 }
 
+func TestFilterOut(t *testing.T) {
+	strs := []string{
+		"1.2.3.4",
+		"",
+		"# 5.6.7.8",
+	}
+
+	want := []string{
+		"1.2.3.4",
+	}
+
+	got := FilterOut(strs, IsCommentOrEmpty)
+	assert.Equal(t, want, got)
+}
+
 func TestInSlice(t *testing.T) {
 	simpleStrs := []string{"1", "2", "3"}