gitweb: Check if requested object exists

gitweb: Check if requested object exists

Try to avoid "Use of uninitialized value ..." errors caused by bad
revision, incorrect filename, wrong object id, bad file etc. (wrong
value of 'h', 'hb', 'f', etc. parameters). This avoids polluting web
server errors log.

Correct git_get_hash_by_path and parse_commit_text (and, in turn,
parse_commit) to return undef if object does not exist.  Check in
git_tag if requested tag exists.

Signed-off-by: Jakub Narebski <jnareb@gmail.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>