Revise @branchHint probabilities

Arguably, my previous use of @setCold() was incorrect in some cases, but
it happened to work out better than not providing any hints.  Let's use
the more granular hints now that Zig 0.14 supports them.

src/bin_export.zig

@@ -118,7 +118,7 @@ pub const Thread = struct {
     }
 
     fn flush(t: *Thread, expected_len: usize) void {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         const block = createBlock(t);
         defer block.deinit();
 

src/json_export.zig

@@ -73,7 +73,7 @@ pub const Writer = struct {
     dir_entry_open: bool = false,
 
     fn flush(ctx: *Writer, bytes: usize) void {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         // This can only really happen when the root path exceeds PATH_MAX,
         // in which case we would probably have error'ed out earlier anyway.
         if (bytes > ctx.buf.len) ui.die("Error writing JSON export: path too long.\n", .{});

src/json_import.zig

@@ -83,7 +83,6 @@ const Parser = struct {
     }
 
     fn fill(p: *Parser) void {
-        @branchHint(.cold);
         p.rdoff = 0;
         p.rdsize = (if (p.zstd) |z| z.read(p.rd, &p.buf) else p.rd.read(&p.buf)) catch |e| switch (e) {
             error.IsDir => p.die("not a file"), // should be detected at open() time, but no flag for that...
@@ -98,6 +97,7 @@ const Parser = struct {
     // (Returning a '?u8' here is nicer but kills performance by about +30%)
     fn nextByte(p: *Parser) u8 {
         if (p.rdoff == p.rdsize) {
+            @branchHint(.unlikely);
             p.fill();
             if (p.rdsize == 0) return 0;
         }
@@ -133,7 +133,7 @@ const Parser = struct {
     }
 
     fn stringContentSlow(p: *Parser, buf: []u8, head: u8, off: usize) []u8 {
-        @branchHint(.cold);
+        @branchHint(.unlikely);
         var b = head;
         var n = off;
         while (true) {