output.diff not always generated; possibly a threading issue

[tbm]

output.diff is sometimes not generated, even though the output has changed:

$ md5sum output/index.html
7c616b023c5f593d57e5b6f5eec1ecbe  output/index.html
$ nanoc compile
Loading site data… done
Compiling site…
      update  [0.02s]  output/index.html

Site compiled in 0.04s.
$ md5sum output/index.html
dfddc976758c15d6c3e3933636aceed3  output/index.html
$ ll output.diff
ls: cannot access output.diff: No such file or directory

I believe this may be a threading issue... when I started adding debug statements using 'warn' in generate_diff_for, the information wasn't always printed.

[tbm]

to reproduce, git clone https://github.com/tbm/homepage
compile
then edit content/index.md and remove the line hide_date: true
compile again and usually there's no output.diff

I added warn statements to generate_diff_for and this info is not always printed·

[denisdefreyne]

This is fixed in #301.