Fix get_default_branch function

[m3adow]

Description

This fixes the get_default_branch_function in the git aliases not working, resulting in several other git aliases not working either.

Motivation and Context

See above.

How Has This Been Tested?

Testing in the bash_it repo:

before:

$ get_default_branch 
Command 'master' not found, did you mean:
  command 'easter' from snap easter (1.2.1)
  command 'master1' from deb pvm-examples (3.4.6-3.2)
See 'snap info <snapname>' for additional versions.
$ gswm
Command 'master' not found, did you mean:
  command 'easter' from snap easter (1.2.1)
  command 'master1' from deb pvm-examples (3.4.6-3.2)
See 'snap info <snapname>' for additional versions.
fatal: missing branch or commit argument

after:

$ get_default_branch 
master
$ gswm
Switched to branch 'master'
Your branch is up to date with 'origin/master'.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• If my change requires a change to the documentation, I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• If I have added a new file, I also added it to clean_files.txt and formatted it using lint_clean_files.sh.
• I have added tests to cover my changes, and all the new and existing tests pass.

It's literally one line changed, so I don't think any tests are needed. I can write some for the function if you want to though.