Get repo default branch if --branch not provided (containerapp up --repo)

[runefa]

---

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• Have you run azdev style <YOUR_EXT> locally? (pip install azdev required)
• Have you run python scripts/ci/test_index.py -q locally?

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[panchagnula]

@StrawnSC please take a look & see if this is something that can be incorporated to --repo / github actions add in general. Please ensure we test this thoroughly before we include this.

[runefa]

Will also need to update the param help @StrawnSC

[panchagnula]

@haroonf & @StrawnSC this change would also need the update to help as well to not say default is "main"

[StrawnSC]

Rewrote this on Calvin's main and added the help text change here: #93

I tested E2E with up and github-actions add