rust: Add a make rustfmt, run it

It is actually really nice that there's One Canonical Style, even
if I sometimes don't like some details of what rustfmt does.

Closes: #1444
Approved by: jlebon

Makefile-rpm-ostree.am

@@ -103,4 +103,8 @@ EXTRA_DIST += $(LIBRPMOSTREE_RUST_SRCS) rust/Cargo.lock
 rpm_ostree_CFLAGS += -Irust/include
 rpm_ostree_SOURCES += rust/include/librpmostree-rust.h
 rpm_ostree_LDADD += $(librpmostree_rust_path)
+rustfmt:
+	rustfmt $(LIBRPMOSTREE_RUST_SRCS)
 endif # ENABLE_RUST
+# Outside the ifdef, otherwise automake complains
+.PHONY: rustfmt

rust/src/treefile.rs

@@ -50,7 +50,9 @@ pub fn treefile_read_impl<W: io::Write>(filename: &Path, output: W) -> io::Resul
 }
 
 fn whitespace_split_packages(pkgs: &[String]) -> Vec<String> {
-    pkgs.iter().flat_map(|pkg| pkg.split_whitespace().map(String::from)).collect()
+    pkgs.iter()
+        .flat_map(|pkg| pkg.split_whitespace().map(String::from))
+        .collect()
 }
 
 #[derive(Serialize, Deserialize, Debug)]