fix(git plugin): check for origin head

[AnatoleLucet]

Standards checklist:

• The PR title is descriptive.
• The PR doesn't replicate another PR which is already open.
• I have read the contribution guide and followed all the instructions.
• The code follows the code style guide detailed in the wiki.
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• The code is stable and I have tested it myself, to the best of my abilities.

Changes:

Adds a condition in git_main_branch to check for the origin remote's HEAD.
Not all repos use master, main, or trunk as their default branch. Some might choose develop, release, etc... This change makes this a bit more dynamic while still keeping the fallbacks for (e.g.) local repos with no origin remote.

Other comments:

I'm not a pro in bash / zsh scripting, so there might be room for improvement here.

[mcornella]

A version of this was released some time ago but removed after complaints that now gcd (git checkout develop) and gcm (git checkout main) did the same thing.

[AnatoleLucet]

Then maybe origin's head could be the fallback if neither master, main, nor trunk exists?

[mcornella]

I appreciate what you're trying to do, but we don't need to support all different configurations. The intent behind git_main_branch was to allow the transition from master to another branch name, and it's done that for the most common name alternatives. It doesn't need to do more.