gitweb: Add support to Link: tag

The tip tree is the one of major subsystem tree in the
Linux kernel project. On the tip tree, the Link: (or
similar Buglink:) tag is used for tracking the original
discussion or context. Since it's ususally in the S-o-b
area, it'd be better using same style with others.

Also as it tends to contain a message-id sent from git
send-email, a part of the line would set a wrong hyperlink
like [1]. Fix it by not using format_log_line_html().

[1] git.kernel.org/?p=linux/kernel/git/tip/tip.git;a=commit;h=08942f6d5d992e9486b07653fd87ea8182a22fa0

Signed-off-by: Namhyung Kim <redacted>
Signed-off-by: Junio C Hamano <redacted>