gitweb: choose appropriate view for file type if a= parameter missing

gitweb: choose appropriate view for file type if a= parameter missing

gitweb URLs use the a= parameter for the view to use on the given path, such
as "blob" or "tree".  Currently, if a gitweb URL omits the a= parameter,
gitweb just shows the top-level repository summary, regardless of the path
given.  gitweb could instead choose an appropriate view based on the file
type: blob for blobs (files), tree for trees (directories), and summary if
no path given (the URL included no f= parameter, or an empty f= parameter).

Apart from making gitweb more robust and supporting URL editing more easily,
this change would aid the creation of shortcuts to git repositories using
simple substitution, such as:
http://example.org/git/?p=path/to/repo.git;hb=HEAD;f=%s

With this patch, if given the hash through the h= parameter, or the hash
base (hb=) and a filename (f=), gitweb uses cat-file -t to automatically set
the a= parameter.

This feature was requested by Josh Triplett through
 http://bugs.debian.org/410465

Signed-off-by: Gerrit Pape <pape@smarden.org>
Signed-off-by: Junio C Hamano <junkio@cox.net>