ui-commit: Preserve path limit in links to commit page

This includes adding a path argument to cgit_commit_link() and updating all
its callers. The callers from within the commit page (i.e. the "commit",
"unidiff"/"side-by-side diff" and "parent" links) all preserve the path
limit of the current commit page. All other callers pass NULL (i.e. no path
limit).

Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Lars Hjemli <hjemli@gmail.com>