write_file: Do not overwrite with same content

The `write_file` function will no longer overwrite existing files if the
content has not changed.  This will ensure file metadata timestamps are
accurate, and reduce unnecessary filesystem activity.

Cargo.lock

@@ -300,6 +300,12 @@ version = "0.4.1"
 source = "registry+https://github.com/rust-lang/crates.io-index"
 checksum = "95505c38b4572b2d910cecb0281560f54b440a19336cbbcb27bf6ce6adc6f5a8"
 
+[[package]]
+name = "hex"
+version = "0.4.3"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "7f24254aa9a54b5c858eaee2f5bccdb46aaf0e486a595ed5fd8f86ba55232a70"
+
 [[package]]
 name = "humansize"
 version = "2.1.3"
@@ -923,6 +929,7 @@ dependencies = [
  "argparse",
  "blake2",
  "file-mode",
+ "hex",
  "pwd-grp",
  "serde",
  "serde_yaml",