cli: add --include-zero flag

This flag, when used in conjunction with --count or --count-matches,
will print a result for each file searched even if there were zero
matches in that file. This is off by default but can be enabled to make
ripgrep behave more like grep.

This also clarifies some of the defaults for the
grep-printer::SummaryBuilder type.

Closes #1370, Closes #1405

tests/misc.rs

@@ -392,6 +392,37 @@ rgtest!(count_matches_via_only, |dir: Dir, mut cmd: TestCommand| {
     eqnice!(expected, cmd.stdout());
 });
 
+rgtest!(include_zero, |dir: Dir, mut cmd: TestCommand| {
+    dir.create("sherlock", SHERLOCK);
+    cmd.args(&[
+        "--count",
+        "--include-zero",
+        "nada",
+    ]);
+    cmd.assert_err();
+
+    let output = cmd.cmd().output().unwrap();
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    let expected = "sherlock:0\n";
+
+    eqnice!(expected, stdout);
+});
+
+rgtest!(include_zero_override, |dir: Dir, mut cmd: TestCommand| {
+    dir.create("sherlock", SHERLOCK);
+    cmd.args(&[
+        "--count",
+        "--include-zero",
+        "--no-include-zero",
+        "nada",
+    ]);
+    cmd.assert_err();
+
+    let output = cmd.cmd().output().unwrap();
+    let stdout = String::from_utf8_lossy(&output.stdout);
+    assert!(stdout.is_empty());
+});
+
 rgtest!(files_with_matches, |dir: Dir, mut cmd: TestCommand| {
     dir.create("sherlock", SHERLOCK);
     cmd.arg("--files-with-matches").arg("Sherlock");