Fix 'git-obs staging group' by fetching the whole branch instead of --depth=1

[dmach]

This fixes failing git hooks and stops git complaining about 'shallow update not allowed'.

Fixes: #2140

[codecov]

Codecov Report

❌ Patch coverage is 0% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 33.93%. Comparing base (b95bc47) to head (39f5cf5).
⚠️ Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
osc/commands_git/staging_group.py	0.00%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #2141   +/-   ##
=======================================
  Coverage   33.93%   33.93%           
=======================================
  Files         199      199           
  Lines       27041    27041           
=======================================
  Hits         9177     9177           
  Misses      17864    17864           

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