[orb init] Change to use gitBranch instead of default master

[bufferings]

I encountered the same issue as #646

By default, it uses the local branch master even though we set other branch name such as main.

So I added a code to use the gitBranch name as the default local branch.

I locally installed the circleci-cli and confirmed it uses the given branch name when An initial commit has been created - please run 'git push origin main' to publish your first commit! message is shown:

❯ git branch
* main

It was master before.

[codecov]

Codecov Report

Merging #678 (fd5ec34) into master (263902a) will decrease coverage by 0.03%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #678      +/-   ##
==========================================
- Coverage   27.37%   27.34%   -0.04%     
==========================================
  Files          41       41              
  Lines        4523     4528       +5     
==========================================
  Hits         1238     1238              
- Misses       3134     3139       +5     
  Partials      151      151              

Impacted Files	Coverage Δ
cmd/orb.go	13.64% <0.00%> (-0.08%)	⬇️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 263902a...fd5ec34. Read the comment docs.