[git] don't set exit code if already set

to avoid overriding manual exit code by processTerminated().

plugins/git4idea/src/git4idea/commands/GitHandler.java

@@ -580,7 +580,12 @@ public abstract class GitHandler {
    * @param exitCode a exit code for process
    */
   protected synchronized void setExitCode(int exitCode) {
-    myExitCode = exitCode;
+    if (myExitCode == null) {
+      myExitCode = exitCode;
+    }
+    else {
+      LOG.info("Not setting exit code " + exitCode + ", because it was already set to " + myExitCode);
+    }
   }
 
   /**