.arcconfig

hgweb: query string arguments to control whitespace for annotate

This feature should hopefully be pretty straightforward. We simply
examine some query string arguments to feed into the diff options.

The function to obtain the diff options has been factored into its
own generic function to facilitate an upcoming change to the HTML
interface and to enable diff settings to be controlled via the same
query string arguments on other web commands.

The test output for "ignoreblanklines" didn't change. I'm not sure
why. Our test coverage for --ignore-blank-lines isn't great and I
can't figure out how to make this diff setting do anything meaningful.
On a very brief examination of the code, it is possible the setting
doesn't work because it is operating at the line level and blank lines
detection needs to examine multiple lines. But I'm not an expert in
this code, so I'm not sure.

Differential Revision: https://phab.mercurial-scm.org/D849

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