Document and test $PATH for SetConfigSearchPaths.

When setting the config search path, the magic string $PATH refers to the
current search path (useful for appending/prepending).

Document and test this behavior in libgit2#.

See:
https://libgit2.github.com/libgit2/#HEAD/group/libgit2/git_libgit2_opts

Author: Ryan Roden-Corrent

LibGit2Sharp.Tests/ConfigurationFixture.cs

@@ -505,6 +505,22 @@ public void CanResetSearchPaths()
             GlobalSettings.SetConfigSearchPaths(ConfigurationLevel.Global, null);
         }
 
+        [Fact]
+        public void CanAppendToSearchPaths()
+        {
+            string appendMe = Path.Combine(Constants.TemporaryReposPath, Path.GetRandomFileName());
+            var prevPaths = GlobalSettings.GetConfigSearchPaths(ConfigurationLevel.Global);
+
+            // append using the special name $PATH
+            GlobalSettings.SetConfigSearchPaths(ConfigurationLevel.Global, "$PATH", appendMe);
+
+            var currentPaths = GlobalSettings.GetConfigSearchPaths(ConfigurationLevel.Global);
+            Assert.Equal(currentPaths, prevPaths.Concat(new[] { appendMe }));
+
+            // set it back to the default
+            GlobalSettings.SetConfigSearchPaths(ConfigurationLevel.Global, null);
+        }
+
         [Fact]
         public void CanRedirectConfigAccess()
         {

LibGit2Sharp/GlobalSettings.cs

@@ -288,7 +288,11 @@ public static IEnumerable<string> GetConfigSearchPaths(ConfigurationLevel level)
         /// Set the paths under which libgit2 searches for the configuration file of a given level.
         /// </summary>
         /// <param name="level">The level (global/system/XDG) of the config.</param>
-        /// <param name="paths">The new search paths, or null to reset to default.</param>
+        /// <param name="paths">
+        ///     The new search paths to set.
+        ///     Pass null to reset to the default.
+        ///     The special string "$PATH" will be substituted with the current search path.
+        /// </param>
         public static void SetConfigSearchPaths(ConfigurationLevel level, params string[] paths)
         {
             var pathString = (paths == null) ? null : string.Join(Path.PathSeparator.ToString(), paths);