Add git_log_link() and fix bug in generic repolink function

The generic repolink function compared head with cgit_query_head, which
almost always would be the same pointer. The test now compares with
repo.defbranch, which is the wanted behavour.

Bug discovered while adding cgit_log_link(), so this commit also contain
that change.

Signed-off-by: Lars Hjemli <hjemli@gmail.com>