feat(reana_dev): create new branch if does not exist in git-checkout (#813)

[jlemesh]

In git-checkout command, if the branch does not exist, create the branch instead of throwing an error.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 1 line in your changes missing coverage. Please review.

Project coverage is 18.15%. Comparing base (8762855) to head (bb666f0).
Report is 23 commits behind head on master.

Files with missing lines	Patch %	Lines
reana/reana_dev/git.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #813      +/-   ##
==========================================
- Coverage   18.15%   18.15%   -0.01%     
==========================================
  Files          26       26              
  Lines        2478     2479       +1     
==========================================
  Hits          450      450              
- Misses       2028     2029       +1     

Files with missing lines	Coverage Δ
reana/reana_dev/git.py	25.30% <0.00%> (-0.05%)	⬇️