Speed up extract_prefix_and_unescape_yaml

On the completions and history thread, the parent function
HistoryFileContents::decode_item() is responsible for ~60% of the CPU time, and
extract_prefix_and_unescape_yaml() alone comprising 14% (of the total).

This change removes allocations in the event that the history item is either
fully or partially plain yaml with no escapes to begin with, and brings down the
execution time of this function to only 7% of the total execution time.

The bulk of the remaining time is spent in wcs2string(), which is called
unconditionally and is naturally alloc-heavy.

src/history/file.rs

@@ -1,6 +1,7 @@
 //! Implemention of history files.
 
 use std::{
+    borrow::Cow,
     fs::File,
     io::{Read, Seek, SeekFrom, Write},
     ops::{Deref, DerefMut},
@@ -312,18 +313,29 @@ fn trim_leading_spaces(s: &[u8]) -> (usize, &[u8]) {
     (count, &s[count..])
 }
 
-fn extract_prefix_and_unescape_yaml(line: &[u8]) -> Option<(Vec<u8>, Vec<u8>)> {
+fn extract_prefix_and_unescape_yaml(line: &[u8]) -> Option<(Cow<[u8]>, Cow<[u8]>)> {
     let mut split = line.splitn(2, |c| *c == b':');
     let key = split.next().unwrap();
     let value = split.next()?;
-    assert!(split.next().is_none());
 
-    let mut key = key.to_owned();
+    let key: Cow<[u8]> = if key.iter().copied().any(|b| b == b'\\') {
+        let mut key = key.to_owned();
+        unescape_yaml_fish_2_0(&mut key);
+        key.into()
+    } else {
+        key.into()
+    };
+
     // Skip a space after the : if necessary.
-    let mut value = trim_start(value).to_owned();
+    let value = trim_start(value);
+    let value: Cow<[u8]> = if value.iter().copied().any(|b| b == b'\\') {
+        let mut value = value.to_owned();
+        unescape_yaml_fish_2_0(&mut value);
+        value.into()
+    } else {
+        value.into()
+    };
 
-    unescape_yaml_fish_2_0(&mut key);
-    unescape_yaml_fish_2_0(&mut value);
     Some((key, value))
 }
 
@@ -361,7 +373,7 @@ fn decode_item_fish_2_0(mut data: &[u8]) -> Option<HistoryItem> {
         // We are definitely going to consume this line.
         data = &data[advance..];
 
-        if key == b"when" {
+        if *key == *b"when" {
             // Parse an int from the timestamp. Should this fail, 0 is acceptable.
             when = time_from_seconds(
                 std::str::from_utf8(&value)
@@ -369,7 +381,7 @@ fn decode_item_fish_2_0(mut data: &[u8]) -> Option<HistoryItem> {
                     .and_then(|s| s.parse().ok())
                     .unwrap_or(0),
             );
-        } else if key == b"paths" {
+        } else if *key == *b"paths" {
             // Read lines starting with " - " until we can't read any more.
             loop {
                 let (advance, line) = read_line(data);