gitweb: Use 's' regexp modifier to secure against filenames with LF

gitweb: Use 's' regexp modifier to secure against filenames with LF

Use 's' (treat string as single line) regexp modifier in
git_get_hash_by_path (against future changes, probably unnecessary)
and in parse_ls_tree_line (when called with '-z'=>1 option) to secure
against filenames containing newline.

[jc: the hunk on git_get_hash_by_path was unneeded, and I noticed the
 regexp was doing unnecessary capture, so fixed it up while I was at it.]

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