Add hash-object to validate ruby hash function

close #1080

Author: jingsam

book/10-git-internals/sections/objects.asc

@@ -381,6 +381,14 @@ You can calculate the SHA-1 value of a string in Ruby by including the SHA1 dige
 => "bd9dbf5aae1a3862dd1526723246b20206e5fc37"
 ----
 
+Let's validate the hash with `git hash-object`. It shows that they are identical. Note that `echo` will add a trailing newline character in string, which is not we want in this case. `echo -n` can prevent from adding a trailing newline character.
+
+[source,console]
+----
+>> $ echo -n "what is up, doc?" | git hash-object --stdin
+=> bd9dbf5aae1a3862dd1526723246b20206e5fc37
+----
+
 Git compresses the new content with zlib, which you can do in Ruby with the zlib library.
 First, you need to require the library and then run `Zlib::Deflate.deflate()` on the content: