Since we were not expecting this config entry to contain a string, we
would fail as soon as its (cached) value would be accessed. Hence,
provide some constants for the 4 states we use, and account for "always"
when we decide to reflog changes.

This commit fixes a regression introduced by

        20a2b02d9a1bcb4825ec49605146223c565dcacf

The commit introduced an optimization for finding references using a
glob: rather than iterating over all references and matching each one
against the glob, we would iterate only over references within the
directory common to all possible references which may match against the
glob.

However, contrary to the `ref/` directory, which was the previous entry
point for the iteration, this directory may not exist. In this case, the
optimization causes an error (`ENOENT`) rather than the iterator simply
yielding no references.

This patch fixes the regression by checkign for this specific case.

Fixes for CVE 2018-11235

Any part of the library which asks the question can pass in the mode to have it
checked against `.gitmodules` being a symlink.

This is particularly relevant for adding entries to the index from the worktree
and for checking out files.

refdb_fs: enhance performance of globbing

A glob used for iteration may start with an entire path containing no
special characters. If we start scanning for references within that path
rather than in `refs/`, we may end up scanning only a small fraction of
all references.

Instead of a hardcoded "refs", we may choose a different directory
within the git directory as the root from which we look for references.

Fix deletion of unrelated branch on worktree

Signed-off-by: Sven Strickroth <email@cs-ware.de>