documentation-file-ref-check: Make git check work for multiple working directories

With multiple git working directories, '.git' may also be a text file
linking to the actual git tree instead of a directory.

Cc: Mauro Carvalho Chehab <mchehab@kernel.org>
Cc: Jonathan Corbet <corbet@lwn.net>
Cc: linux-doc@vger.kernel.org
Signed-off-by: Rob Herring <robh@kernel.org>
Reviewed-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Tested-by: Mauro Carvalho Chehab <mchehab@kernel.org>
Link: https://lore.kernel.org/r/20210603012806.331132-1-robh@kernel.orgSigned-off-by: Jonathan Corbet <corbet@lwn.net>

scripts/documentation-file-ref-check

@@ -24,7 +24,7 @@ my $help = 0;
 my $fix = 0;
 my $warn = 0;
 
-if (! -d ".git") {
+if (! -e ".git") {
 	printf "Warning: can't check if file exists, as this is not a git tree\n";
 	exit 0;
 }