fix: compute ref and ref_name prioritising HEAD pointer if tag is not set

[GROwen]

Fixes nektos/act#2056

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 83.33333% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 61.22%. Comparing base (4989f44) to head (7a878b3). Report is 365 commits behind head on master.

Files	Patch %	Lines
pkg/common/git/git.go	83.33%	2 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2061      +/-   ##
==========================================
- Coverage   61.22%   61.22%   -0.01%     
==========================================
  Files          46       53       +7     
  Lines        7141     8755    +1614     
==========================================
+ Hits         4372     5360     +988     
- Misses       2462     2970     +508     
- Partials      307      425     +118     

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.

[mergify[bot]]

@GROwen this pull request is now in conflict 😩