move Repository.Pull to Worktree.Pull

[mcuadros]

This PR moves the method Pull from the Repository to Worktree, this method should be there because is only useful when the repository is not bare. This is the behavior at cgit:

> git clone git clone [email protected]:git-fixtures/basic.git --bare
....     
> git pull                            
fatal: This operation must be run in a work tree

[smola]

s/Pull/Fetch

[codecov]

Codecov Report

Merging #499 into master will decrease coverage by 0.51%.
The diff coverage is 62.06%.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
- Coverage   77.99%   77.48%   -0.52%     
==========================================
  Files         127      127              
  Lines        9402     9375      -27     
==========================================
- Hits         7333     7264      -69     
- Misses       1253     1318      +65     
+ Partials      816      793      -23

Impacted Files	Coverage Δ
repository.go	74.22% <52.94%> (+0.96%)	⬆️
worktree.go	66.25% <75%> (+0.7%)	⬆️
plumbing/transport/ssh/common.go	2.81% <0%> (-45.08%)	⬇️
plumbing/transport/ssh/auth_method.go	33.33% <0%> (-24.77%)	⬇️
remote.go	72.13% <0%> (+1%)	⬆️

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