[PATCH] contrib: GnomeKeyring support + generic helper implementation

All,

the following patch series proposes enhancements to the credential helper implementations in the contrib section. The detailed development history can be found at GitHub [1].

  The first patch adds a GnomeKeyring credential backend.  The GnomeKeyring
specific parts are based on the work by John Szakmeister, who wrote the
helper originally for the initial, unpublished version of the credential
helper protocol.

  The second patch adds a generic implementation of a credential helper
based on a simplified credential API and common helper functions.  Helpers
based on this implementation do not need to worry about the credential
protocol and only need to implement callback functions for the different
credential operations.

  The third and fourth patches port the existing helpers to this generic
implementation.

Looking forward to your thoughts.

Greetings from Oldenburg,
  Philipp

[1] https://github.com/pah/git-credential-helper