Git - v2.38.2

This is to backport various fixes accumulated during the development
towards Git 2.39, the next feature release.

Fixes since v2.38.1

• Update CodingGuidelines to clarify what features to use and avoid
  in C99.

• The codepath that reads from the index v4 had unaligned memory
  accesses, which has been corrected.

• "git remote rename" failed to rename a remote without fetch
  refspec, which has been corrected.

• "git clone" did not like to see the "--bare" and the "--origin"
  options used together without a good reason.

• Fix messages incorrectly marked for translation.

• "git fsck" failed to release contents of tree objects already used
  from the memory, which has been fixed.

• "git rebase -i" can mistakenly attempt to apply a fixup to a commit
  itself, which has been corrected.

• In read-only repositories, "git merge-tree" tried to come up with a
  merge result tree object, which it failed (which is not wrong) and
  led to a segfault (which is bad), which has been corrected.

• Force C locale while running tests around httpd to make sure we can
  find expected error messages in the log.

• Fix a logic in "mailinfo -b" that miscomputed the length of a
  substring, which lead to an out-of-bounds access.

• The codepath to sign learned to report errors when it fails to read
  from "ssh-keygen".

• "GIT_EDITOR=: git branch --edit-description" resulted in failure,
  which has been corrected.

• Documentation on various Boolean GIT_* environment variables have
  been clarified.

• "git multi-pack-index repack/expire" used to repack unreachable
  cruft into a new pack, which have been corrected.

• The code to clean temporary object directories (used for
  quarantine) tried to remove them inside its signal handler, which
  was a no-no.

• "git branch --edit-description" on an unborh branch misleadingly
  said that no such branch exists, which has been corrected.

• GitHub CI settings have been adjusted to recent reality, merging
  and cherry-picking necessary topics that have been prepared for Git
  2.39.

• git rebase --update-refs would delete references when all update-ref
  commands in the sequencer were removed, which has been corrected.

Also contains various documentation updates and code clean-ups.