Fix "reply" link in Differential

Summary:
Fixes the issue caused by rPa0af5b66437719dba6136579c051982ab275e6a0. Prior to
that patch, isCommentInNewFile() returned $comment->getIsNewFile(). While this
was often the wrong value, it came from the database and was the integer 1 if
true.

After the patch, the function returns 'true' as a boolean, which is passed to JS
and then back to PHP, interpreted as an integer, and evaluates to 0.

To avoid this issue in general, provide an isBool() method on AphrontRequest
which interprets this correctly.

I will also revert the revert of rPa0af5b66437719dba6136579c051982ab275e6a0 when
I land this.

Test Plan:
Clicked "reply" on the right hand side of a diff, got a right-hand-side inline
comment.

Reviewed By: rm
Reviewers: tuomaspelkonen, jungejason, aran, rm
CC: simpkins, aran, epriestley, rm
Differential Revision: 250