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

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

Further work to 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-more:
  git-compat-util: fix CONST_OUTPARAM typo and indentation
  refs/files-backend: drop const to fix strchr() warning
  http: drop const to fix strstr() warning
  range-diff: drop const to fix strstr() warnings
  pkt-line: make packet_reader.line non-const
  skip_prefix(): check const match between in and out params
  pseudo-merge: fix disk reads from find_pseudo_merge()
  find_last_dir_sep(): convert inline function to macro
  run-command: explicitly cast away constness when assigning to void
  pager: explicitly cast away strchr() constness
  transport-helper: drop const to fix strchr() warnings
  http: add const to fix strchr() warnings
  convert: add const to fix strchr() warnings