examples: add example for pulling changes

[orirawlings]

A question came up in #545 about how to conduct the equivalent of git pull with the library. I suspect that the appropriate method was overlooked at first because it is under the Worktree type instead of the Repository type (where the Clone/Fetch/Push methods live).

This PR adds some example code to demonstrate getting the Worktree for a repository and issuing a Pull from the origin remote.

[codecov]

Codecov Report

Merging #546 into master will decrease coverage by 0.6%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #546      +/-   ##
==========================================
- Coverage   78.08%   77.48%   -0.61%     
==========================================
  Files         129      129              
  Lines        9798     9798              
==========================================
- Hits         7651     7592      -59     
- Misses       1316     1388      +72     
+ Partials      831      818      -13

Impacted Files	Coverage Δ
plumbing/transport/ssh/common.go	20.54% <0%> (-45.21%)	⬇️
plumbing/transport/ssh/auth_method.go	31.57% <0%> (-22.81%)	⬇️

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