perf: optimize update_plugin function

[Dabolus]

Summary

This PR improves the performance of the update_plugin function by only contacting remote if/when needed.
It does so by making two main changes:

1. Only running the ls-remote when the third parameter to the function is not provided, as before it was always run and its result was ignored if the third parameter was there;
2. Avoiding to fetch remote unless needed, i.e. not fetching if the provided gitref is a commit and is already available locally.

This gives a big performance boost, especially when on a slow internet connection, and removes the dependency on remote when it's not needed. For example, the change is very noticeable when re-running add-all while using asdf-plugin-manager.

Benchmarks

With the original implementation:

• asdf plugin update <plugin-name>: ≈3.5s ← remote contacted twice
• asdf plugin update <plugin-name> <branch>: ≈3.5s ← remote contacted twice
• asdf plugin update <plugin-name> <new gitref>: ≈3.5s ← remote contacted twice
• asdf plugin update <plugin-name> <existing gitref>: ≈3.5s ← remote contacted twice

With these changes:

• asdf plugin update <plugin-name>: ≈3.5s ← remote contacted twice
• asdf plugin update <plugin-name> <branch>: ≈1.7s ← remote contacted once
• asdf plugin update <plugin-name> <new gitref>: ≈1.7s ← remote contacted once
• asdf plugin update <plugin-name> <existing gitref>: ≈0.1s ← remote not contacted at all

Note: these benchmarks are from my machine with my internet connection. Results may vary a lot depending on the internet connection of the device running them.