Allow recursing into specific folders

[maxfischer2781]

In order to select a specific subfolder, one currently needs to prepare a top-level config with include statements or enable full --recurse and blacklist/whitelist things. These are quite some hoops to jump through for what seems like an obvious use-case.

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--recurse could take an optional list of directories to recurse into:

• Use top-level configs only:
  condor-git-config https://github.com/foo/bar

• Use top-level and nested configs:
  condor-git-config https://github.com/foo/bar --recurse

• Use top-level and nested configs in ./qux:
  condor-git-config https://github.com/foo/bar --recurse qux

Alternatively, we could take one or several --base (or similar) folders in which to search:

• Use top-level configs only:
  condor-git-config https://github.com/foo/bar

• Use top-level and nested configs in ./qux:
  condor-git-config https://github.com/foo/bar --base . qux

• Use nested configs in ./qux:
  condor-git-config https://github.com/foo/bar --base qux