gitweb: refactor input parameters parse/validation

gitweb: refactor input parameters parse/validation

Since input parameters can be obtained both from CGI parameters and
PATH_INFO, we would like most of the code to be agnostic about the way
parameters were retrieved. We thus collect all the parameters into the
new %input_params hash, delaying validation after the collection is
completed.

Although the kludge removal is minimal at the moment, it makes life much
easier for future expansions such as more extensive PATH_INFO use or
other form of input such as command-line support.

Signed-off-by: Giuseppe Bilotta <redacted>
Acked-by: Jakub Narebski <redacted>
Signed-off-by: Shawn O. Pearce <redacted>