scripts/generate-template.sh: Fix long standing typo

Also, while we are there, change the link to use https.
scripts/generate-template.sh: Fix diff calculation

The old way to calculate the diff wasn't working well and it can be totally
simplified by letting git calculate the diff (git is pretty well at that ;)).

So this patch just overwrites the old template with the new one, asks git for
the diff, and then mail it.
Merge pull request #44 from rata/master

scripts/generate-template.sh: Fix long standing typo and diff