Encapsulate request handling in CGitServer

No functional change but makes the code slightly nicer.

Signed-off-by: John Keeping <john@keeping.me.uk>
Return None if a configured path is empty

Signed-off-by: John Keeping <john@keeping.me.uk>
Make InstaCGit application class

No functional change but this will make future changes simpler.

Signed-off-by: John Keeping <john@keeping.me.uk>
If cgit.script isn't specified, fallback to datadir

This makes things slightly simpler for poeple running CGit from a
checkout of the Git repository.

Signed-off-by: John Keeping <john@keeping.me.uk>
Improve detection of CGit

Extract the error messasge to make the code cleaner and check that the
files exist as well as being specified.

Signed-off-by: John Keeping <john@keeping.me.uk>
Extract cgitrc content to a constant

This makes the indentation more consistent by avoiding putting the
multiline string into the middle of a function.

Signed-off-by: John Keeping <john@keeping.me.uk>