mod: read file from argument for diff contents

For example you can run the following bash command:

```bash
tui-diff <(git diff)
```

With this command you create a temporary file containing
contents of the `git diff` command and is provided to
`tui-diff` to open and render.

src/main.zig

@@ -120,6 +120,13 @@ pub fn main() !void {
         else => area.allocator(),
     };
 
+    var threaded_io: std.Io.Threaded = .init(allocator, .{});
+    errdefer threaded_io.deinit();
+    const io = threaded_io.io();
+
+    var diff_input: [:0]u8 = undefined;
+    defer allocator.free(diff_input);
+
     // argument handling
     {
         var arg_it = try std.process.argsWithAllocator(allocator);
@@ -127,21 +134,39 @@ pub fn main() !void {
 
         // TODO may there be other options?
         // usage: tui-diff
-
         // skip own executable name
         _ = arg_it.skip();
+
+        // only handle the first argument otherwise ignore!
+        if (arg_it.next()) |file| {
+            var buffer: [4096]u8 = undefined;
+            var stream = std.Io.File.readerStreaming(
+                try std.Io.Dir.openFileAbsolute(
+                    io,
+                    file,
+                    .{ .mode = .read_only },
+                ),
+                io,
+                &buffer,
+            );
+            const reader = &stream.interface;
+            diff_input = try reader.allocRemainingAlignedSentinel(
+                allocator,
+                .unlimited,
+                .of(u8),
+                0,
+            );
+        } else {
+            // TODO detect VCS in the current working directory (and traversing upwards if none found at point?)
+        }
     }
     // tui creation
     errdefer |err| log.err("Application Error: {any}", .{err});
 
-    var threaded_io: std.Io.Threaded = .init(allocator, .{});
-    errdefer threaded_io.deinit();
-    const io = threaded_io.io();
-
     var renderer = zterm.Renderer.Buffered.init(allocator);
     defer renderer.deinit();
 
-    var app: App = .init(io, try .init(allocator, diff));
+    var app: App = .init(io, try .init(allocator, diff_input));
     defer app.model.deinit(allocator);
 
     var element_root: tui_diff.elements.Root(App) = .init(allocator);