arch-pkg-repo-updater does not update repos after initial clone

So as it turns out that we observed stale information in bumpbuddy that this was due to the change to arch-pkg-repo-updater, since the tool only does the equivalent of a a git fetch after the initial clone:

$ git status
On branch main
Your branch is behind 'origin/main' by 1 commit, and can be fast-forwarded.
  (use "git pull" to update your local branch)

nothing to commit, working tree clean

Manually updating produces the expected result:

$ git pull
Updating 4192364..5f2d411
Fast-forward
 .SRCINFO | 6 +++---
 PKGBUILD | 4 ++--
 2 files changed, 5 insertions(+), 5 deletions(-)

I think that this is an issue for arch-pkg-repo-updater aswell as the usage in buildbtw itself 🤔

Checking the git2-rs examples, it shouldn't be too hard to implement the missing logic for a pull: https://github.com/rust-lang/git2-rs/blob/master/examples/pull.rs

cc @antiz