scan-tree: Unify gitweb.* and cgit.* settings into one config option.

After some back and forth with Jamie and René, it looks like the git
config semantics are going to be like this:

- gitweb.category maps to the cgit repo config key "section"
- gitweb.description maps to the cgit repo config key "desc"
- gitweb.owner maps to the cgit repo config key "owner"
- cgit.* maps to all cgit repo config keys

This option can be enabled with "enable-git-config=1", and replaces
all previous "enable-gitweb-*" config keys.

The order of operations is as follows:

- git config settings are applied in the order that they exist in
  the git config file
- if the owner is not set from git config, get the owner using the
  usual getpwuid call
- if the description is not set from git config, look inside the
  static $path/description file
- if section-from-path=1, override whatever previous settings were
  inside of git config using the section-from-path logic
- parse $path/cgitrc for local repo.* settings, that override all
  previous settings