util: add a function to show the mount point of the filesystem

For now, this is Windows only, since Linux doesn't have the value in its statfs
structure, and I don't have a BSD system to test with.

mercurial/posix.py

@@ -305,6 +305,13 b' def checkosfilename(path):'
     Returns None if the path is ok, or a UI string describing the problem.'''
     return None # on posix platforms, every path is ok
 
+def getfsmountpoint(dirpath):
+    '''Get the filesystem mount point from a directory (best-effort)
+
+    Returns None if we are unsure. Raises OSError on ENOENT, EPERM, etc.
+    '''
+    return getattr(osutil, 'getfsmountpoint', lambda x: None)(dirpath)
+
 def getfstype(dirpath):
     '''Get the filesystem type name from a directory (best-effort)
 

mercurial/util.py

@@ -109,6 +109,7 b' executablepath = platform.executablepath'
 expandglobs = platform.expandglobs
 explainexit = platform.explainexit
 findexe = platform.findexe
+getfsmountpoint = platform.getfsmountpoint
 getfstype = platform.getfstype
 gethgcmd = platform.gethgcmd
 getuser = platform.getuser

mercurial/windows.py

@@ -32,6 +32,7 b' except ImportError:'
 osutil = policy.importmod(r'osutil')
 
 executablepath = win32.executablepath
+getfsmountpoint = win32.getvolumename
 getfstype = win32.getfstype
 getuser = win32.getuser
 hidewindow = win32.hidewindow