commit hashes are monospaced

Hi, I've noticed the commit hashes don't use monospaced fonts, so lists of commits look ugly. This change sets them to the monospaced fonts, so they look like this:

app/assets/stylesheets/generic/gfm.scss

@@ -19,3 +19,7 @@
height: 14em;
}
}
+
+.gfm-commit, .gfm-commit_range {
+  font-family: $monospace_font;
+}