Allow boolean flags to be repeated

bat prepends the arguments from its config file to the command line, so a
user who has `--show-all` in their config and also types `-A` passes the
flag twice, and clap rejects that with "the argument '--show-all' cannot be
used multiple times" and exit code 2. The file is never displayed.

`--plain`, `--number`, `--number-nonblank`, `--force-colorization` and
`--no-paging` already avoid this by overriding themselves. The same is now
done for the eight remaining flags that affect how a file is displayed:
--show-all, --diff, --chop-long-lines, --squeeze-blank, --unbuffered,
--quiet-empty, --no-custom-assets and --set-terminal-title.

Repeating a flag produces byte-identical output to giving it once, and
--show-all still conflicts with --language as before.

tests/examples/bat-show-all.conf

@@ -0,0 +1 @@
+--show-all

tests/integration_tests.rs

@@ -646,6 +646,30 @@ fn piped_output_with_default_style_flag() {
         );
 }
 
+#[test]
+fn repeated_boolean_flag_is_accepted() {
+    // A flag in the config file is prepended to the command line, so a user
+    // who has `--show-all` in their config and also types `-A` ends up passing
+    // it twice. That must not be an error.
+    bat()
+        .arg("empty_lines.txt")
+        .arg("--show-all")
+        .arg("--show-all")
+        .assert()
+        .success();
+}
+
+#[test]
+fn boolean_flag_in_config_and_on_command_line() {
+    bat_with_config()
+        .env("BAT_CONFIG_PATH", "bat-show-all.conf")
+        .arg("empty_lines.txt")
+        .arg("--show-all")
+        .arg("--paging=never")
+        .assert()
+        .success();
+}
+
 #[test]
 fn squeeze_blank() {
     bat()