Using RC4 is not a _certificate_ problem, it's a cipher problem.  The
SSL implementation should and will fail with an unrecoverable error
(-1).  There's no opportunity to accept/continue.

This used to fail with an error indicating a mis-use of OpenSSL on platforms
using it due to poor error handling. Re-enable it even if this isn't the right
error code to use for now.

Ensure that we created `refs/remotes/origin/HEAD` when cloning, a
symbolic link pointing to `refs/remotes/origin/<default>`

httpclient: support googlesource

We _dispose_ the contents of objects; we _free_ objects (and their
contents).  Update `git_strarray_free` to be `git_strarray_dispose`.
`git_strarray_free` remains as a deprecated proxy function.

Google Git (googlesource.com) behaves differently than git proper.
Test that we can communicate with it.

In commit b9c5b15a7 (http: use the new httpclient, 2019-12-22), the HTTP
code got refactored to extract a generic HTTP client that operates
independently of the Git protocol. Part of refactoring was the creation
of a new `git_http_request` struct that encapsulates the generation of
requests. Our Git-specific HTTP transport was converted to use that in
`generate_request`, but during the process we forgot to set up custom
headers for the `git_http_request` and as a result we do not send out
these headers anymore.

Fix the issue by correctly setting up the request's custom headers and
add a test to verify we correctly send them.