Add support for repo-local cgitrc file

Add support for repo-local cgitrc file

When recursively scanning a directory tree looking for git repositories,
cgit will now parse cgitrc files found within such repositories.

The repo-specific config files can include any repo-specific options
except 'repo.url' and 'repo.path'. Also, in such config files the 'repo.'
prefix can not be used, i.e. the valid options then becomes:
* name
* clone-url
* desc
* ower
* defbranch
* snapshots
* enable-log-filecount
* enable-log-linecount
* max-stats
* module-link
* section
* about-filter
* commit-filter
* source-filter
* readme

Signed-off-by: Lars Hjemli <hjemli@gmail.com>