gitweb: Fix two 'uninitialized value' warnings in git_tree()

gitweb: Fix two 'uninitialized value' warnings in git_tree()

If we did try to access nonexistent directory or file, which means
that git_get_hash_by_path() returns `undef`, uninitialized $hash
variable was passed to 'open' call.  Now we fail early with "404 Not
Found - No such tree" error.  (If we try to access something which
does not resolve to tree-ish, for example a file / 'blob' object, the
error will be caught later, as "404 Not Found - Reading tree failed"
error).

If we tried to use 'tree' action without $file_name ('f' parameter)
set, which means either tree given by hash or a top tree (and we
currently cannot distinguish between those two cases), we cannot print
path breadcrumbs with git_print_page_path().  Fix this by moving call
to git_print_page_path() inside conditional.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Acked-by: Petr Baudis <pasky@suse.cz>
Signed-off-by: Shawn O. Pearce <spearce@spearce.org>