gitweb: Do not show 'patch' link for merge commits

gitweb: Do not show 'patch' link for merge commits

The 'patch' view is about generating text/plain patch that can be
given to "git am", and "git am" doesn't understand merges anyway.
Therefore link to 'patch' view should not be shown for merge commits.

Also call to git-format-patch inside the 'patch' action would fail
when 'patch' action is called for a merge commit, with "Reading
git-format-patch failed" text as 'patch' view body.

Signed-off-by: Jakub Narebski <redacted>
Signed-off-by: Junio C Hamano <redacted>