Merge branch 'ss/rename-tests'

Reorganize some tests and rename them; "ls t/" now gives a better
overview of what is tested for these scripts than before.

* ss/rename-tests:
  t7501: rename commit test to comply with naming convention
  t7500: rename commit tests script to comply with naming convention
  t7502: rename commit test script to comply with naming convention
  t7509: cleanup description and filename
  t2000: rename and combine checkout clash tests