gitweb: add patch view

gitweb: add patch view

The output of commitdiff_plain is not intended for git-am:
 * when given a range of commits, commitdiff_plain publishes a single
   patch with the message from the first commit, instead of a patchset
 * the hand-built email format replicates the commit summary both as
   email subject and as first line of the email itself, resulting in
   a duplication if the output is used with git-am.

We thus create a new view that can be fed to git-am directly, allowing
patch exchange via gitweb. The new view exposes the output of git
format-patch directly, limiting it to a single patch in the case of a
single commit.

A configurable upper limit defaulting to 16 is imposed on the number of
commits which will be included in a patchset, to prevent DoS attacks on
the server. Setting the limit to 0 will disable the patch view, setting
it to a negative number will remove the limit.

Signed-off-by: Giuseppe Bilotta <giuseppe.bilotta@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>