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