* Scope config snapshot to files inside the working tree
* Record excluded snapshot entries as placeholders instead of links
* Limit linked snapshot content to unmodified tracked files and tracked directories
File targets reached through a link are included only when their content is
unchanged from HEAD, and directory targets only when they contain tracked
files; anything else is recorded as a single placeholder. Adds tests for a
sensitive path that links to a tracked directory, links to untracked
directories, and links to tracked files modified after checkout.
e1fc925862