gitweb: Add 'show-sizes' feature to show blob sizes in tree view

gitweb: Add 'show-sizes' feature to show blob sizes in tree view

Add support for 'show-sizes' feature to show (in separate column,
between mode and filename) the size of blobs (files) in the 'tree'
view.  It passes '-l' option to "git ls-tree" invocation.

For the 'tree' and 'commit' (submodule) entries, '-' is shown in place
of size; for generated '..' "up directory" entry nothing is shown.

The 'show-sizes' feature is enabled by default.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>