Use test -h instead of test -L for portability

Citing from the GNU autoconf manual:
"either form conforms to Posix 1003.1-2001, but older shells like Solaris 8
 /bin/sh support only -h"

tests/test-merge-types

@@ -16,7 +16,7 b' hg merge'
 
 echo % symlink is left parent, executable is right
 
-if [ -L a ]; then
+if [ -h a ]; then
     echo a is a symlink
     readlink a
 elif [ -x a ]; then
@@ -28,7 +28,7 b' hg merge'
 
 echo % symlink is right parent, executable is left
 
-if [ -L a ]; then
+if [ -h a ]; then
     echo a is a symlink
     readlink a
 elif [ -x a ]; then