Cache time string of the last mtime

Files can have the same mtime (especially when using
`SOURCE_DATE_EPOCH`). Cache the time string of the last mtime.

Benchmark results on a Raspberry Pi Zero 2W running Ubuntu 24.04 (noble)
arm64:

```
$ sudo 3cpio -x /boot/initrd.img -C /var/tmp/initrd
$ ( cd /var/tmp/initrd && find . | LC_ALL=C sort | sudo cpio --reproducible --quiet -o -H newc ) > initrd.img
$ hyperfine -N --warmup 3 --runs 100 "target/release/3cpio -tv initrd.img" "3cpio-0.3.0+9 -tv initrd.img"
Benchmark 1: target/release/3cpio -tv initrd.img
  Time (mean ± σ):     108.8 ms ±   1.1 ms    [User: 46.3 ms, System: 61.3 ms]
  Range (min … max):   107.1 ms … 111.5 ms    100 runs

Benchmark 2: 3cpio-0.3.0+9 -tv initrd.img
  Time (mean ± σ):     113.2 ms ±   1.0 ms    [User: 51.2 ms, System: 60.8 ms]
  Range (min … max):   111.4 ms … 115.7 ms    100 runs

Summary
  target/release/3cpio -tv initrd.img ran
    1.04 ± 0.01 times faster than 3cpio-0.3.0+9 -tv initrd.img
```

Signed-off-by: Benjamin Drung <bdrung@posteo.de>

src/lib.rs

@@ -476,6 +476,10 @@ fn read_cpio_and_print_long_format<R: Read + SeekForward, W: Write>(
     now: i64,
     user_group_cache: &mut UserGroupCache,
 ) -> Result<()> {
+    // Files can have the same mtime (especially when using SOURCE_DATE_EPOCH).
+    // Cache the time string of the last mtime.
+    let mut last_mtime = 0;
+    let mut time_string: String = "".into();
     loop {
         let header = match read_cpio_header(file) {
             Ok(header) => {
@@ -497,6 +501,10 @@ fn read_cpio_and_print_long_format<R: Read + SeekForward, W: Write>(
             None => header.gid.to_string(),
         };
         let mode_string = header.mode_string();
+        if header.mtime != last_mtime || time_string.is_empty() {
+            last_mtime = header.mtime;
+            time_string = format_time(header.mtime, now)?;
+        };
 
         if header.mode & MODE_FILETYPE_MASK == FILETYPE_SYMLINK {
             let target = header.read_symlink_target(file)?;
@@ -508,7 +516,7 @@ fn read_cpio_and_print_long_format<R: Read + SeekForward, W: Write>(
                 user,
                 group,
                 header.filesize,
-                format_time(header.mtime, now)?,
+                time_string,
                 header.filename,
                 target
             )?;
@@ -522,7 +530,7 @@ fn read_cpio_and_print_long_format<R: Read + SeekForward, W: Write>(
                 user,
                 group,
                 header.filesize,
-                format_time(header.mtime, now)?,
+                time_string,
                 header.filename
             )?;
         };