readd update flag to cp command (#10824)

# Description
- this PR should close #10819


# User-Facing Changes
Behaviour is similar to pre 0.86.0 behaviour of the cp command and
should as such not have a user-facing change, only compared to the
current version, were the option is readded.


# After Submitting
I guess the documentation will be automatically updated and as this
feature is no further highlighted, probably, no more work will be needed
here.

# Considerations
coreutils actually allows a third option:
```
pub enum UpdateMode {
    // --update=`all`,
    ReplaceAll,
    // --update=`none`
    ReplaceNone,
    // --update=`older`
    // -u
    ReplaceIfOlder,
}
```
namely `ReplaceNone`, which I have not added. Also I think that
specifying `--update 'abc'` is non functional.

crates/nu-command/src/filesystem/ucp.rs

@@ -7,7 +7,7 @@ use nu_protocol::{
     Category, Example, PipelineData, ShellError, Signature, Spanned, SyntaxShape, Type,
 };
 use std::path::PathBuf;
-use uu_cp::{BackupMode, UpdateMode};
+use uu_cp::{BackupMode, CopyMode, UpdateMode};
 
 // TODO: related to uucore::error::set_exit_code(EXIT_ERR)
 // const EXIT_ERR: i32 = 1;
@@ -46,6 +46,11 @@ impl Command for UCp {
                 Some('f'),
             )
             .switch("interactive", "ask before overwriting files", Some('i'))
+            .switch(
+                "update",
+                "copy only when the SOURCE file is newer than the destination file or when the destination file is missing",
+                Some('u')
+            )
             .switch("progress", "display a progress bar", Some('p'))
             .switch("no-clobber", "do not overwrite an existing file", Some('n'))
             .switch("debug", "explain how a file is copied. Implies -v", None)
@@ -76,6 +81,11 @@ impl Command for UCp {
                 example: "cp *.txt dir_a",
                 result: None,
             },
+            Example {
+                description: "Copy only if source file is newer than target file",
+                example: "cp -u a b",
+                result: None,
+            },
         ]
     }
 
@@ -87,6 +97,11 @@ impl Command for UCp {
         _input: PipelineData,
     ) -> Result<PipelineData, ShellError> {
         let interactive = call.has_flag("interactive");
+        let (update, copy_mode) = if call.has_flag("update") {
+            (UpdateMode::ReplaceIfOlder, CopyMode::Update)
+        } else {
+            (UpdateMode::ReplaceAll, CopyMode::Copy)
+        };
         let force = call.has_flag("force");
         let no_clobber = call.has_flag("no-clobber");
         let progress = call.has_flag("progress");
@@ -198,7 +213,7 @@ impl Command for UCp {
             backup: BackupMode::NoBackup,
             copy_contents: false,
             cli_dereference: false,
-            copy_mode: uu_cp::CopyMode::Copy,
+            copy_mode,
             no_target_dir: false,
             one_file_system: false,
             parents: false,
@@ -207,7 +222,7 @@ impl Command for UCp {
             attributes: uu_cp::Attributes::NONE,
             backup_suffix: String::from("~"),
             target_dir: None,
-            update: UpdateMode::ReplaceAll,
+            update,
         };
 
         if let Err(error) = uu_cp::copy(&sources, &target_path, &options) {

crates/nu-command/tests/commands/cp.rs

@@ -582,7 +582,6 @@ fn copy_file_with_read_permission_impl(progress: bool) {
     });
 }
 
-#[ignore = "not implemented with ucp"]
 #[test]
 fn copy_file_with_update_flag() {
     copy_file_with_update_flag_impl(false);