Fix build.get_repo() to return only directory that contains 'repodata/repomd.xml'

[dmach]

Fixes: #477

[codecov]

Codecov Report

Patch coverage: 11.11% and no project coverage change.

Comparison is base (e25dc2d) 28.81% compared to head (2cdf052) 28.82%.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1307   +/-   ##
=======================================
  Coverage   28.81%   28.82%           
=======================================
  Files          36       36           
  Lines       16639    16637    -2     
=======================================
+ Hits         4794     4795    +1     
+ Misses      11845    11842    -3     

Impacted Files	Coverage Δ
osc/build.py	4.73% <0.00%> (+0.01%)	⬆️
osc/core.py	35.66% <ø> (ø)
osc/commandline.py	19.04% <100.00%> (+0.01%)	⬆️

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[dmach]

Checked with @lethliel who reported the issue and he thinks the patch should be ok. Merging...