Improve o2checkcode

[vkucera]

• Allow auto-detect of .clang-tidy. (Report if found.)
• Remove explicit list of Clang-Tidy checks.
  • Configuration is taken from .clang-tidy if found. (Requires Update Clang-Tidy config file AliceO2Group/AliceO2#15321)
  • Additional check configuration from O2_CHECKER_CHECKS is appended via the -checks command line option. (Report the provided value.)
• Print enabled checks before running instead of all available.
• Don't turn warnings into errors. Report both.
• Fix unbound ALIBUILD_BASE_HASH.

FIXME: git diff can fail silently, leading to an early exit due to an empty list of changed files. This is currently happening in the alidist fullCI check.