[FIXED JENKINS-13007]

On Windows command prompt, '^' is an escape character (http://en.wikipedia.org/wiki/Escape_character#Windows_Command_Prompt)
This isn't a problem if 'git' we are executing is git.exe, because '^' is a special character only for the command processor,
but if 'git' we are executing is git.cmd (which is the case of msysgit), then the arguments we pass in here ends up getting
processed by the command processor, and so 'xyz^{commit}' becomes 'xyz{commit}' and fails.

Since we can't really tell if we are calling into git.exe or git.cmd, the best we can do for Windows
is not to use '^{commit}'. This reverts 13f6038
and it will not dereference tags, but it's far better than having this method completely broken.

See JENKINS-13007 where this blew up on Windows users.

I filed msysgit/msysgit#36 as a bug in msysgit.

src/main/java/hudson/plugins/git/GitAPI.java

@@ -257,7 +257,21 @@ public void clean() throws GitException {
     }
 
     public ObjectId revParse(String revName) throws GitException {
-        String rpCommit = Functions.isWindows() ? "^^{commit}" : "^{commit}";
+        /*
+            On Windows command prompt, '^' is an escape character (http://en.wikipedia.org/wiki/Escape_character#Windows_Command_Prompt)
+            This isn't a problem if 'git' we are executing is git.exe, because '^' is a special character only for the command processor,
+            but if 'git' we are executing is git.cmd (which is the case of msysgit), then the arguments we pass in here ends up getting
+            processed by the command processor, and so 'xyz^{commit}' becomes 'xyz{commit}' and fails.
+
+            Since we can't really tell if we are calling into git.exe or git.cmd, the best we can do for Windows
+            is not to use '^{commit}'. This reverts 13f6038acc4fa5b5a62413155da6fc8cfcad3fe0
+            and it will not dereference tags, but it's far better than having this method completely broken.
+
+            See JENKINS-13007 where this blew up on Windows users.
+
+            I filed https://github.com/msysgit/msysgit/issues/36 as a bug in msysgit.
+         */
+        String rpCommit = Functions.isWindows() ? "" : "^{commit}";
         String result = launchCommand("rev-parse", revName + rpCommit);
         return ObjectId.fromString(firstLine(result).trim());
     }