support structured branch name

[fcollonval]

Fix branch names like ft/foo or fix/bar

This happens there for example: https://github.com/fcollonval/lumino/runs/3163528043?check_suite_focus=true

+ echo 'Using GITHUB_REF: refs/heads/ft/accordion-panel'
Using GITHUB_REF: refs/heads/ft/accordion-panel
++ cut -d/ -f 3
++ echo refs/heads/ft/accordion-panel
+ export RH_BRANCH=ft
+ RH_BRANCH=ft

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[codecov-commenter]

Codecov Report

Merging #89 (dacd923) into master (84235d3) will decrease coverage by 2.55%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##           master      #89      +/-   ##
==========================================
- Coverage   87.83%   85.27%   -2.56%     
==========================================
  Files          15       16       +1     
  Lines        1759     1915     +156     
  Branches      213      241      +28     
==========================================
+ Hits         1545     1633      +88     
- Misses        134      191      +57     
- Partials       80       91      +11     

Impacted Files	Coverage Δ
jupyter_releaser/actions/draft_release.py	0.00% <0.00%> (ø)
jupyter_releaser/lib.py	74.28% <54.54%> (-0.43%)	⬇️
jupyter_releaser/util.py	69.87% <55.55%> (-12.32%)	⬇️
jupyter_releaser/tee.py	82.05% <82.05%> (ø)
jupyter_releaser/tests/test_cli.py	96.54% <82.75%> (-1.11%)	⬇️
jupyter_releaser/__init__.py	100.00% <100.00%> (ø)
jupyter_releaser/cli.py	92.59% <100.00%> (+0.21%)	⬆️
jupyter_releaser/tests/test_functions.py	100.00% <100.00%> (ø)

---

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