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 <[e-mail]>
Cc: Jonathan Corbet <[e-mail]>
Cc: [e-mail]
Signed-off-by: Rob Herring <[e-mail]>
Reviewed-by: Mauro Carvalho Chehab <[e-mail]>
Tested-by: Mauro Carvalho Chehab <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Jonathan Corbet <[e-mail]>

scripts/documentation-file-ref-check

···
 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;
 }