git.py(git_cmd): always pass "-c" "fetch.prune=false" to git

[glacambre]

Problem: Anod sometimes breaks because users set fetch.prune=true in
their .gitconfig, which causes Git to remove references that are
actually required by Anod during git fetches.

Reproducer: Add the below snippet of code to your .gitconfig and run
anod update twice.

[fetch]
  prune = true

Solution: Make every git command run with -c fetch.prune=false. This
ensures that every git command will behave in an unsurprising way
with regards to reference pruning on fetch. When pruning references on
fetch is actually required, users of the git_cmd command will be
able to add --prune to their fetch subcommand.

[CLAassistant]

All committers have signed the CLA.

[glacambre]

@Nikokrock Sorry to bother you, but I'm not sure who would be the right reviewer for this PR. Could you have a look? (Also sorry for pinging you rather that setting the proper PR metadata, it looks like I don't have enough rights to do this.)