v0.13.0 — hgit version

hgit version closes doc 09's own explicitly-flagged gap: no version
string was embedded in the packaged file itself.

• Hgit.HC defines HGIT_VERSION "0.13.0" - matches this release tag
  exactly, the first real test of that discipline.
• hgit version prints HGIT_VERSION 0.13.0.
• hgit help now shows HELP_VERSION 0.13.0 as its own first line.
• Verified on real QEMU; regression (experiments/65-head-deletion's
  full command-surface test) re-run clean.
• packaging/HgitAll.HC, 151,258 bytes, tools/lint-package.sh clean.

See experiments/74-hgit-version/.