Don't load changesets when IssuesController#show processes API requests without "include=changesets"
Currently, changesets are always being loaded in IssuesController when API requests for IssuesController#show are made. However, loading Changesets is not necessary unless "include=changesets" is specified in the API request parameters.
The attached patch can improve the performance of "GET /issues/:id.(json|xml)" by loading changesets only when they are necessary.