gitweb: Modularized git_get_project_description to be more generic

gitweb: Modularized git_get_project_description to be more generic

Introduce a git_get_file_or_project_config utility function to
retrieve a repository variable either from a plain text file in the
$GIT_DIR or else from 'gitweb.$variable' in the repository config
(e.g. 'description').

This would be used in next commit to retrieve category for a project,
which is to be stored in the same way as project description.

Signed-off-by: Sebastien Cevey <redacted>
Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>