watch git-annex program file to detect upgrades

Not yet wired up to restart the assistant on upgrade; that needs careful
sanity checking to wait until the upgrade is done before restarting.

Used the DirWatcher here, so it gets events for any changes to the
directory containing the program file. (But not subdirs.) This is necessary
in order to detect when the file is renamed as part of the upgrade, which
an inotify on a single file would not detect. (Also, I have DirWatcher code,
but not FileWatcher code.)

Note that upgrades that remove or rename a whole directory tree containing
the executable will *not* trigger this code. So eg, deleting and replacing
the whole standalone tarball dir tree won't work -- but untarring it
over top will. So should dpkg package upgrades.

Added programPath, using a new GHC feature to find the full path to the
executable. The fallback code for old GHC or unsupported OS is less good;
its worst failure mode would be either failing to find the program, and so
not checking for upgrades, or finding a git-annex that's in PATH, but is
not the one running.

This commit was sponsored by John Roepke.

Config/Files.hs

@@ -5,11 +5,14 @@
  - Licensed under the GNU GPL version 3 or higher.
  -}
 
+{-# LANGUAGE CPP #-}
+
 module Config.Files where
 
 import Common
 import Utility.Tmp
 import Utility.FreeDesktop
+import System.Environment
 
 {- ~/.config/git-annex/file -}
 userConfigFile :: FilePath -> IO FilePath
@@ -67,3 +70,23 @@ readProgramFile = do
 		)
   where
   	cmd = "git-annex"
+
+{- A fully qualified path to the currently running git-annex program.
+ - 
+ - getExecutablePath is available since ghc 7.4.2. On OSs it supports
+ - well, it returns the complete path to the program. But, on other OSs,
+ - it might return just the basename.
+ -}
+programPath :: IO (Maybe FilePath)
+programPath = do
+#if MIN_VERSION_base(4,6,0)
+	exe <- getExecutablePath
+	p <- if isAbsolute exe
+		then return exe
+		else readProgramFile
+#else
+	p <- readProgramFile
+#endif
+	-- In case readProgramFile returned just the command name,
+	-- fall back to finding it in PATH.
+	searchPath p