gitweb: extend blame to show links to diff and previous

gitweb: extend blame to show links to diff and previous

git_blame2() now has two more columns, "Prev" and "Diff",
before the "Commit" column, as follows:

Prev	Diff	Commit	Line	Data
SHA     Diff    SHA        N    ...
...

The "Prev" column shows the SHA of the parent commit,
between which this line changed.  Clicking on it shows the
blame of the file as of the parent commit, for that line.

So clicking repeatedly on "Prev" would show you the blame
of that file, from the point of view of the changes
of that particular line whose "Prev" you're clicking on.

The "Diff" column shows "Diff" which is a link to blobdiff
between "Prev" and "Commit" commits _for that line_.

So clicking on "Diff" would show you the blobdiff (HTML)
between the parent commit and this commit which changed
that particular line.

Signed-off-by: Luben Tuikov <ltuikov@yahoo.com>
Signed-off-by: Junio C Hamano <junkio@cox.net>