Merge branch 'jk/c23-const-preserving-fixes'

Adjust the codebase for C23 that changes functions like strchr()
that discarded constness when they return a pointer into a const
string to preserve constness.

* jk/c23-const-preserving-fixes:
  config: store allocated string in non-const pointer
  rev-parse: avoid writing to const string for parent marks
  revision: avoid writing to const string for parent marks
  rev-parse: simplify dotdot parsing
  revision: make handle_dotdot() interface less confusing