Add hydration percentages to git status hook

[tyrielv]

For a few releases, git status would output a message like "You are in a partially-hydrated checkout with 1% of tracked files present."

However, this implementation only measured files that had been modified; files that were hydrated but unmodified were not included, so the change was reverted.

This change adds a line to the start of git status similar to "1% of files and 1% of folders hydrated. Run 'gvfs health' for details."

I decided to show separate %s for files and folders because I think there is a meaningful difference between "0% files and 100% folders" and "10% files and 10% folders", but if combined both could display as "11% total hydration" (with ~1:10 ratio of folders to files).

** Changes **

1. A new option is added to gvfs health verb, --status, to display the new output line.
2. Gvfs.Hooks.exe will call gvfs health --status on pre-command hook for git status except when called with --serialize.
3. The hydration %s are calculated from:
   • the number of files/folders in placeholder database
   • the number of files/folders in modified database
   • the total number of files, from git index header
   • the total number of folders, from git ls-tree -r -d HEAD
4. Because the git ls-tree command can be slow for large repos, its result is cached.
   • GitStatusCache is modified log the hydration stats to telemetry, which ensures the cache is up-to-date.
   • The new cache file is stored in a new file TreeCountCache.dat next to the existing GitStatusCache.dat.
   • The cache is keyed off the tree ID of HEAD.
5. Refactoring: Reading the placeholder, modified DB file/folder lists, and index file/folder lists are moved from HealthVerb to EnlistmentPathData
6. If the repo is not mounted, gvfs health [--status] will attempt to read the modified file list directly instead of getting it from the current mount process.