RFR: cli: add git-proxy tool

[Erik Helin]

Hi all,

please review this patch that adds a small new tool: `git-proxy`. `git-proxy`
can be used to set up the `HTTPS_PROXY` and `HTTP_PROXY` environment
variables based on either the `http.proxy` configuration setting or the `-c
http.proxy=` argument. Finally `git-proxy` will also set up the
`GIT_SSH_COMMAND` environment variable to use the `CONNECT` method for tunneling
SSH over an HTTP proxy.

Testing:
- manual testing of `git-proxy`
- `make test` passes on Linux x64

Thanks,
Erik

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Commit messages:
 - cli: add git-proxy tool

Changes: https://git.openjdk.java.net/skara/pull/585/files
 Webrev: https://webrevs.openjdk.java.net/skara/585/webrev.00
  Stats: 158 lines in 3 files changed: 157 ins; 0 del; 1 mod
  Patch: https://git.openjdk.java.net/skara/pull/585.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/585/head:pull/585

PR: https://git.openjdk.java.net/skara/pull/585