Make serve and watch respect .gitignore

When editing files with some editors (e.g. vim) temporary files are
constantly being modified causing mdbook serve and watch to constantly
rebuild the project.

To avoid this leverage the ignore crate to load the project's root
.gitignore file and skip changes on files ignored.

Cargo.toml

@@ -21,6 +21,7 @@ clap = "2.24"
 env_logger = "0.6"
 error-chain = "0.12"
 handlebars = { version = "2.0", default-features = false, features = ["no_dir_source"] }
+ignore = "0.1.8"
 itertools = "0.8"
 lazy_static = "1.0"
 log = "0.4"

src/cmd/serve.rs

@@ -6,6 +6,7 @@ use iron::{status, AfterMiddleware, Chain, Iron, IronError, IronResult, Request,
 use mdbook::errors::*;
 use mdbook::utils;
 use mdbook::MDBook;
+use std::path::PathBuf;
 
 struct ErrorRecover;
 
@@ -59,6 +60,7 @@ pub fn make_subcommand<'a, 'b>() -> App<'a, 'b> {
                 .help("Port to use for WebSockets livereload connections"),
         )
         .arg_from_usage("-o, --open 'Opens the book server in a web browser'")
+        .arg_from_usage("-i, --gitignore 'Respect .gitignore and skip changes to ignored files'")
 }
 
 // Watch command implementation
@@ -107,8 +109,16 @@ pub fn execute(args: &ArgMatches) -> Result<()> {
         open(serving_url);
     }
 
+    let gitignore_path: Option<PathBuf> = if args.is_present("gitignore") {
+        let mut path = book_dir.clone();
+        path.push(".gitignore");
+        Some(path)
+    } else {
+        None
+    };
+
     #[cfg(feature = "watch")]
-    watch::trigger_on_change(&book, move |paths, book_dir| {
+    watch::trigger_on_change(&book, gitignore_path, move |paths, book_dir| {
         info!("Files changed: {:?}", paths);
         info!("Building book...");