Adds a number of user-accessable options for configuring internal git resource
cloning behavior.
- Git commands are executed with a configurable timeout by including a parameter
like "?timeout=2m30s" in the resource URL. This can improve cloning a large
repository, or over a slow network.
- Git submodule cloning can be disabled by including a parameter like
"?submodules=false" in the resource URL.
- Switch the overall query parsing to use url.Parse() and be more extensible.
Currently, we always fetch the default branch with an initial git clone
and then fetch the ref after if it's specified. This changes it to only
make one fetch instead of two if a ref is specified.
