RFR: git-fork: support subsection specific configuration

[Erik Helin]

Hi all,

please review this patch that allows for subsection specific configurations for `git-fork`. 
For example if a user wants to reference different local repositories for
different upstream projects, that is now possible:

[fork]
       ssh = true
       sync = true

[fork "openjdk/jdk"]
       reference = ~/git/github.com/openjdk/jdk

[fork "openjdk/skara"]
       reference = ~/git/github.com/openjdk/skara

Thanks,
Erik

- [x] Manual testing of `git-fork`

-------------

Commits:
 - db9f8011: git-fork: support subsection specific configuration

Changes: https://git.openjdk.java.net/skara/pull/305/files
 Webrev: https://webrevs.openjdk.java.net/skara/305/webrev.00
  Stats: 65 lines in 1 file changed: 33 ins; 22 del; 10 mod
  Patch: https://git.openjdk.java.net/skara/pull/305.diff
  Fetch: git fetch https://git.openjdk.java.net/skara pull/305/head:pull/305

PR: https://git.openjdk.java.net/skara/pull/305