Git repositories should always be updated when the ref is a branch.
They should be also updated if no ref was specified or if the ref isn’t
present. This stops the repo being updated if we’re building a specific
revision and it is present. Additionally, this stops the update barfing
on repositories where we’re using a revision and there is no upstream
`master` branch.

Closes Homebrew/homebrew#29218.

Fixes Homebrew/homebrew#28187.

We know what strategy we want, so going through DownloadStrategyDetector
is wasted work. Now we can remove those patterns from the detector and
have two fewer branches each time through.

Closes Homebrew/homebrew#26871.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

This should be "resource", not "resources", though it works anyway
because there is a also "resource" reader method.

Closes Homebrew/homebrew#25751.
Closes Homebrew/homebrew#26730.

Fixes Homebrew/homebrew#22159.

Closes Homebrew/homebrew#24775.

Fixes Homebrew/homebrew#24157.

Fixes Homebrew/homebrew#24444.
Closes Homebrew/homebrew#24445.
Closes Homebrew/homebrew#24458.

A better fix would be to rewrite the curl method to yield its output,
but I don't feel like doing that right now.

Fixes Homebrew/homebrew#23764.

Ruby's OpenURI library is somewhat broken under 1.8 and chokes on
otherwise valid values of http(s)_proxy. Use curl to get the mirror list
instead.

Fixes Homebrew/homebrew#23708.