feat(pack): prefer using revision from lockfile during install

Problem: Installing plugin always pulls latest `version` changes
  (usually from the default branch or "latest version tag"). It is more
  robust to prefer initial installation to use the latest recorded
  (i.e. "working") revision.

Solution: Prefer using revision from the lockfile (if present) during
  install. The extra `update()` will pull the latest changes.

runtime/doc/pack.txt

@@ -224,8 +224,9 @@ tags following semver convention `v<major>.<minor>.<patch>`.
 The latest state of all managed plugins is stored inside a *vim.pack-lockfile*
 located at `$XDG_CONFIG_HOME/nvim/nvim-pack-lock.json`. It is a JSON file that
 is used to persistently track data about plugins. For a more robust config
-treat lockfile like its part: put under version control, etc. Should not be
-edited by hand or deleted.
+treat lockfile like its part: put under version control, etc. In this case
+initial install prefers revision from the lockfile instead of inferring from
+`version`. Should not be edited by hand or deleted.
 
 Example workflows ~
 
@@ -260,7 +261,8 @@ Basic install and management:
     plugin1 = require('plugin1')
 <
 • Restart Nvim (for example, with |:restart|). Plugins that were not yet
-  installed will be available on disk in target state after `add()` call.
+  installed will be available on disk after `add()` call. Their revision is
+  taken from |vim.pack-lockfile| (if present) or inferred from the `version`.
 • To update all plugins with new changes:
   • Execute |vim.pack.update()|. This will download updates from source and
     show confirmation buffer in a separate tabpage.