posix: move checkexec 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.

mercurial/posix.py

@@ -160,7 +160,10 b' def checkexec(path):'
 
     try:
         EXECFLAGS = stat.S_IXUSR | stat.S_IXGRP | stat.S_IXOTH
-        fh, fn = tempfile.mkstemp(dir=path, prefix='hg-checkexec-')
+        cachedir = os.path.join(path, '.hg', 'cache')
+        if not os.path.isdir(cachedir):
+            cachedir = path
+        fh, fn = tempfile.mkstemp(dir=cachedir, prefix='hg-checkexec-')
         try:
             os.close(fh)
             m = os.stat(fn).st_mode & 0o777