Merge branch 'tz/doc-txt-to-adoc-fixes'

Merge branch 'tz/doc-txt-to-adoc-fixes'

Fallouts from recent renaming of documentation files from .txt
suffix to the new .adoc suffix have been corrected.

* tz/doc-txt-to-adoc-fixes: (38 commits)
  xdiff: *.txt -> *.adoc fixes
  unpack-trees.c: *.txt -> *.adoc fixes
  transport.h: *.txt -> *.adoc fixes
  trace2/tr2_sysenv.c: *.txt -> *.adoc fixes
  trace2.h: *.txt -> *.adoc fixes
  t6434: *.txt -> *.adoc fixes
  t6012: *.txt -> *.adoc fixes
  t/helper/test-rot13-filter.c: *.txt -> *.adoc fixes
  simple-ipc.h: *.txt -> *.adoc fixes
  setup.c: *.txt -> *.adoc fixes
  refs.h: *.txt -> *.adoc fixes
  pseudo-merge.h: *.txt -> *.adoc fixes
  parse-options.h: *.txt -> *.adoc fixes
  object-name.c: *.txt -> *.adoc fixes
  list-objects-filter-options.h: *.txt -> *.adoc fixes
  fsck.h: *.txt -> *.adoc fixes
  diffcore.h: *.txt -> *.adoc fixes
  diff.h: *.txt -> *.adoc fixes
  contrib/long-running-filter: *.txt -> *.adoc fixes
  config.c: *.txt -> *.adoc fixes
  ...