Fix Windows issue `INDEX remote origin must have url as a local file`

• Fix issue #12 causing the below error with romt crate import on Windows:

  INDEX remote ``origin`` must have ``url`` as a local file
  

Additional changes:

• Clarify quick-start instructions, pointing out steps which are one-time only.

• Add probe for Alpine Linux's default location for git-http-backend
  (/usr/libexec/git-core/git-http-backend) in addition to the more common
  location (/usr/lib/git-core/git-http-backend). This eliminates the need
  for manual configuration with Alpine (see issue #11).

• Improve exception messages. For DownloadError in particular, embed the
  repr() of the associated exception from httpx to aid in debugging
  httpx-related errors (see also issue #10 for more debugging of httpx
  proxy-related issues).