Support absoulte path in git submodule gitdirs

When using git submodules, some versions of git (e.g., 1.7.9) use
abosulate path in the gitdir reference of the submodules. This patch
adds support to handle absoluate path correctly.

Author: maiergre

src/main/java/pl/project13/maven/git/GitDirLocator.java

@@ -46,7 +46,7 @@ public File lookupGitDirectory(@NotNull File manuallyConfiguredDir) {
 
     if (manuallyConfiguredDir.exists()) {
 
-      // If manuallyConfiguredDir is a directory then we can use it as the git path. 
+      // If manuallyConfiguredDir is a directory then we can use it as the git path.
       if (manuallyConfiguredDir.isDirectory()) {
         return manuallyConfiguredDir;
       }
@@ -158,7 +158,14 @@ private File processGitDirFile(@NotNull File file) {
         }
 
         // All seems ok so return the "gitdir" value read from the file.
-        return new File(file.getParentFile(), parts[1]);
+        File gitDir = new File(parts[1]);
+        if (gitDir.isAbsolute()) {
+          // gitdir value is an absolute path. Return as-is
+          return gitDir;
+        } else {
+          // gitdir value is relative.
+          return new File(file.getParentFile(), parts[1]);
+        }
       } catch (FileNotFoundException e) {
         return null;
       } finally {