versioning: Expose .revision-hash as $ZSH_HIGHLIGHT_REVISION.

Currently, this only works in installed trees and exported tarballs; the
parameter is left unset when running directly from a git worktree.

zsh-syntax-highlighting.zsh

@@ -32,6 +32,13 @@
 if [[ -o function_argzero ]]; then
   # $0 is reliable
   ZSH_HIGHLIGHT_VERSION=$(<"$0:h"/.version)
+  ZSH_HIGHLIGHT_REVISION=$(<"$0:h"/.revision-hash)
+  if [[ $ZSH_HIGHLIGHT_REVISION == \$Format:* ]]; then
+    # When running from a source tree without 'make install', $ZSH_HIGHLIGHT_REVISION
+    # would be set to '$Format:%H$' literally.  That's an invalid value, and obtaining
+    # the valid value (via `git rev-parse HEAD`, as Makefile does) might be costly, so:
+    unset ZSH_HIGHLIGHT_REVISION
+  fi
 else
   # $0 is unreliable, so the call to _zsh_highlight_load_highlighters will fail.
   # TODO: If 'zmodload zsh/parameter' is available, ${funcsourcetrace[1]%:*} might serve as a substitute?