After creating a few repos in github, initializing them and running into the same problem I decided to figure out how to fix it.

Here’s the message:

You asked me to pull without telling me which branch you
want to merge with, and 'branch.master.merge' in
your configuration file does not tell me either.  Please
name which branch you want to merge on the command line and
try again (e.g. 'git pull <repository> <refspec>').
See git-pull(1) for details on the refspec.

If you often merge with the same branch, you may want to

configure the following variables in your configuration file:
branch.master.remote = 
branch.master.merge = 
remote..url = 
remote..fetch = 
See git-config(1) for details.

You can get around doing config changes by adding additional parameters to the git pull command:

git pull origin master

But who wants to do that everytime? Not me. Especially since I have my aliases setup:

alias ga='git add'
alias gb='git branch'
alias gba='git branch -a'
alias gc='git commit -v'
alias gca='git commit -v -a'
alias gd='git diff'
alias gko='git checkout'
alias gl='git pull --rebase'
alias glv='git pull -v'
alias gp='git push'
alias gpv='git push -v'
alias gs='git stash'
alias gst='git status'

So the right thing to do is fix this by modifying .git/config in your repo and adding the following section:

[branch "master"]
  remote = origin
  merge = refs/heads/master

Now I can use my aliases as I like and know a little more about git.

It’s a process.