Over time, the http_get_strbuf function has grown several
optional parameters. We now have a bitfield with multiple
boolean options, as well as an optional strbuf for returning
the content-type of the response. And a future patch in this
series is going to add another strbuf option.

Treating these as separate arguments has a few downsides:

  1. Most call sites need to add extra NULLs and 0s for the
     options they aren't interested in.

  2. The http_get_* functions are actually wrappers around
     2 layers of low-level implementation functions. We have
     to pass these options through individually.

  3. The http_get_strbuf wrapper learned these options, but
     nobody bothered to do so for http_get_file, even though
     it is backed by the same function that does understand
     the options.

Let's consolidate the options into a single struct. For the
common case of the default options, we'll allow callers to
simply pass a NULL for the options struct.

The resulting code is often a few lines longer, but it ends
up being easier to read (and to change as we add new
options, since we do not need to update each call site).
Signed-off-by: NJeff King <peff@peff.net>
Signed-off-by: NJonathan Nieder <jrnieder@gmail.com>