RFR: build: respect git's http.proxy setting

[Erik Helin]

Hi all,

please review this small patch that makes the `gradlew` script respect git's
`http.proxy` setting.

Testing:
- Manual testing of `sh gradlew`

Thanks,
Erik

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Commit messages:
 - build: respect git's http.proxy setting

Changes: https://git.openjdk.java.net/skara/pull/586/files
 Webrev: https://webrevs.openjdk.java.net/skara/586/webrev.00
  Stats: 11 lines in 1 file changed: 11 ins; 0 del; 0 mod
  Patch: https://git.openjdk.java.net/skara/pull/586.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/586/head:pull/586

PR: https://git.openjdk.java.net/skara/pull/586