gitweb: Enable tree (directory) history display

gitweb: Enable tree (directory) history display

This patch allows history display of whole trees/directories a la
"git-rev-list HEAD -- <dir or file>".  I find this useful especially
when a project lives in its own subdirectory, as opposed to being all
of the GIT repository (i.e. when a sub-project is merged into a
super-project).

Signed-off-by: Luben Tuikov <redacted>
Signed-off-by: Junio C Hamano <redacted>