posix: move checklink test file to .hg/cache

posix: move checklink test file to .hg/cache

This avoids unnecessary churn in the working directory.

It is not necessarily a fully valid assumption that .hg/cache is on the same
filesystem as the working directory, but I think it is an acceptable
approximation. It could also be the case that different parts of the working
directory is on different mount points so checking in the root folder could
also be wrong.

--- a/mercurial/posix.py	Wed Jan 14 01:15:26 2015 +0100
+++ b/mercurial/posix.py	Thu Nov 17 12:59:36 2016 +0100
@@ -219,9 +219,16 @@
     # mktemp is not racy because symlink creation will fail if the
     # file already exists
     while True:
-        name = tempfile.mktemp(dir=path, prefix='hg-checklink-')
+        cachedir = os.path.join(path, '.hg', 'cache')
+        if os.path.isdir(cachedir):
+            checkdir = cachedir
+        else:
+            checkdir = path
+            cachedir = None
+        name = tempfile.mktemp(dir=checkdir, prefix='checklink-')
         try:
-            fd = tempfile.NamedTemporaryFile(dir=path, prefix='hg-checklink-')
+            fd = tempfile.NamedTemporaryFile(dir=checkdir,
+                                             prefix='hg-checklink-')
             try:
                 os.symlink(os.path.basename(fd.name), name)
                 os.unlink(name)