apt-get update only for a specific repository

yes, apt-get can do that, and can do it in a nice way.

1. Append following to ~/.bash_funcs

   update-repo() {
       for source in "$@"; do
           sudo apt-get update -o Dir::Etc::sourcelist="sources.list.d/${source}" \
           -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0"    
       done
   }
   

2. Append following to ~/.bashrc

   if [ -f $HOME/.bash_funcs ]; then
   .  $HOME/.bash_funcs
   fi
   

3. Append following to ~/.bash_completion

   # Debian user-defined completion                             -*- shell-script -*-
   
   _ppa_lists(){
       local cur
       _init_completion || return
   
       COMPREPLY=( $( find /etc/apt/sources.list.d/ -name "*$cur*.list" \
   -exec basename {} \; 2> /dev/null ) )
       return 0
   } &&
   complete -F _ppa_lists update-repo
   

4. Then source the files

   . ~/.bashrc
   . ~/.bash_completion
   

5. Done and start to fire it

   update-repo <tab> <tab>
   

You can update a single ppa repository without having to update whole apt source, with implement of bash-completion.

If the repository is configured in a specific file in the directory /etc/apt/sources.list.d/, say myrepo.list, you can update that single repository with the command:

sudo apt-get update -o Dir::Etc::sourcelist="sources.list.d/myrepo.list" \
    -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0"

Nevertheless this is not very convenient.
This can be simplified defining a bash function

update_repo() {
    sudo apt-get update -o Dir::Etc::sourcelist="sources.list.d/$1.list" \
        -o Dir::Etc::sourceparts="-" -o APT::Get::List-Cleanup="0"
}

so that you can simply run

update_repo myrepo

Y PPA Manager comes with a command line tool called update-ppa that lets you update a single PPA.

For example:

sudo update-ppa ppa:nilarimogard/webupd8

Also, when adding a PPA through Y PPA Manager, the PPA source is automatically updated (only for that PPA). In a future version, there's going to be a GUI to manually update single PPAs as well.

More information about Y PPA Manager, HERE.