Update default branch logic

[rohitvinnakota-codecov]

Purpose/Motivation

What is the feature? Why is this being done?

We are seeing some issues in the UI where the default branch of main cannot be fetched. Since the git community uses main as default, we are updating our backend to reflect that.

This PR is a follow up from https://github.com/codecov/shared/pull/289

Legal Boilerplate

Look, I get it. The entity doing business as "Sentry" was incorporated in the State of Delaware in 2015 as Functional Software, Inc. In 2022 this entity acquired Codecov and as result Sentry is going to need some rights from me in order to utilize my contributions in this PR. So here's the deal: I retain all rights, title and interest in and to my contributions, and by keeping this boilerplate intact I confirm that Sentry can use, modify, copy, and redistribute my contributions, under Sentry's choice of terms.

[codecov-notifications[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

:white_check_mark: All tests successful. No failed tests found.

:loudspeaker: Thoughts on this report? Let us know!

[codecov-qa[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.71%. Comparing base (4a45617) to head (2ce8927).

:white_check_mark: All tests successful. No failed tests found.

@@           Coverage Diff           @@
##             main     #681   +/-   ##
=======================================
  Coverage   91.71%   91.71%           
=======================================
  Files         632      632           
  Lines       17039    17039           
=======================================
  Hits        15628    15628           
  Misses       1411     1411           

Flag	Coverage Δ
unit	91.71% <ø> (ø)
unit-latest-uploader	91.71% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/managers.py	0.00% <ø> (ø)

:mega: Codecov offers a browser extension for seamless coverage viewing on GitHub. Try it in Chrome or Firefox today!

[codecov-public-qa[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.71%. Comparing base (4a45617) to head (2ce8927).

:white_check_mark: All tests successful. No failed tests found.

@@           Coverage Diff           @@
##             main     #681   +/-   ##
=======================================
  Coverage   91.71%   91.71%           
=======================================
  Files         632      632           
  Lines       17039    17039           
=======================================
  Hits        15628    15628           
  Misses       1411     1411           

Flag	Coverage Δ
unit	91.71% <ø> (ø)
unit-latest-uploader	91.71% <ø> (ø)

Flags with carried forward coverage won't be shown. Click here to find out more.

Files	Coverage Δ
core/managers.py	0.00% <ø> (ø)

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.01%. Comparing base (4a45617) to head (2ce8927).

:white_check_mark: All tests successful. No failed tests found.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #681 +/- ## =========================================== Coverage 96.01000 96.01000 =========================================== Files 814 814 Lines 18390 18390 =========================================== Hits 17658 17658 Misses 732 732 ``` | [Flag](https://app.codecov.io/gh/codecov/codecov-api/pull/681/flags?src=pr&el=flags&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codecov) | Coverage Δ | | |---|---|---| | [unit](https://app.codecov.io/gh/codecov/codecov-api/pull/681/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codecov) | `91.71% <ø> (ø)` | | | [unit-latest-uploader](https://app.codecov.io/gh/codecov/codecov-api/pull/681/flags?src=pr&el=flag&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codecov) | `91.71% <ø> (ø)` | | Flags with carried forward coverage won't be shown. [Click here](https://docs.codecov.io/docs/carryforward-flags?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=codecov#carryforward-flags-in-the-pull-request-comment) to find out more.

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[rohitvinnakota-codecov]

Merging this on Monday, July 29 as we have a scheduled audit then.