fsck: warn about symlink pointing inside a gitdir

In the wake of fixing a vulnerability where `git clone` mistakenly
followed a symbolic link that it had just written while checking out
files, writing into a gitdir, let's add some defense-in-depth by
teaching `git fsck` to report symbolic links stored in its trees that
point inside `.git/`.

Even though the Git project never made any promises about the exact
shape of the `.git/` directory's contents, there are likely repositories
out there containing symbolic links that point inside the gitdir. For
that reason, let's only report these as warnings, not as errors.
Security-conscious users are encouraged to configure
`fsck.symlinkPointsToGitDir = error`.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

Documentation/fsck-msgids.txt

@@ -157,6 +157,18 @@
 `nullSha1`::
 	(WARN) Tree contains entries pointing to a null sha1.
 
+`symlinkPointsToGitDir`::
+	(WARN) Symbolic link points inside a gitdir.
+
+`symlinkTargetBlob`::
+	(ERROR) A non-blob found instead of a symbolic link's target.
+
+`symlinkTargetLength`::
+	(WARN) Symbolic link target longer than maximum path length.
+
+`symlinkTargetMissing`::
+	(ERROR) Unable to read symbolic link target's blob.
+
 `treeNotSorted`::
 	(ERROR) A tree is not properly sorted.