I hope this is useful to someone. I was trying to set up git and gitweb.

>sudo apt-get install git-core

was successful but gitweb was not,

>sudo apt-get install gitweb
Reading package lists… Done
Building dependency tree
Reading state information… Done
Package gitweb is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package gitweb has no installation candidate

If you get this error message,

Try sudo apt-get update

and then run

sudo apt-get install gitweb

This installed gitweb successfully.