git-p4: fix empty file processing for large file system backend GitLFS

If git-p4 tried to store an empty file in GitLFS then it crashed while
parsing the pointer file:

  oid = re.search(r'^oid \w+:(\w+)', pointerFile, re.MULTILINE).group(1)
  AttributeError: 'NoneType' object has no attribute 'group'

This happens because GitLFS does not create a pointer file for an empty
file. Teach git-p4 this behavior to fix the problem and add a test case.

Signed-off-by: Lars Schneider <larsxschneider@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-p4.py

@@ -1005,18 +1005,20 @@ class LargeFileSystem(object):
            steps."""
         if self.exceedsLargeFileThreshold(relPath, contents) or self.hasLargeFileExtension(relPath):
             contentTempFile = self.generateTempFile(contents)
-            (git_mode, contents, localLargeFile) = self.generatePointer(contentTempFile)
-
-            # Move temp file to final location in large file system
-            largeFileDir = os.path.dirname(localLargeFile)
-            if not os.path.isdir(largeFileDir):
-                os.makedirs(largeFileDir)
-            shutil.move(contentTempFile, localLargeFile)
-            self.addLargeFile(relPath)
-            if gitConfigBool('git-p4.largeFilePush'):
-                self.pushFile(localLargeFile)
-            if verbose:
-                sys.stderr.write("%s moved to large file system (%s)\n" % (relPath, localLargeFile))
+            (pointer_git_mode, contents, localLargeFile) = self.generatePointer(contentTempFile)
+            if pointer_git_mode:
+                git_mode = pointer_git_mode
+            if localLargeFile:
+                # Move temp file to final location in large file system
+                largeFileDir = os.path.dirname(localLargeFile)
+                if not os.path.isdir(largeFileDir):
+                    os.makedirs(largeFileDir)
+                shutil.move(contentTempFile, localLargeFile)
+                self.addLargeFile(relPath)
+                if gitConfigBool('git-p4.largeFilePush'):
+                    self.pushFile(localLargeFile)
+                if verbose:
+                    sys.stderr.write("%s moved to large file system (%s)\n" % (relPath, localLargeFile))
         return (git_mode, contents)
 
 class MockLFS(LargeFileSystem):
@@ -1056,6 +1058,9 @@ class GitLFS(LargeFileSystem):
            the actual content. Return also the new location of the actual
            content.
            """
+        if os.path.getsize(contentFile) == 0:
+            return (None, '', None)
+
         pointerProcess = subprocess.Popen(
             ['git', 'lfs', 'pointer', '--file=' + contentFile],
             stdout=subprocess.PIPE

t/t9824-git-p4-git-lfs.sh

@@ -42,6 +42,8 @@ test_expect_success 'Create repo with binary files' '
 	(
 		cd "$cli" &&
 
+		>file0.dat &&
+		p4 add file0.dat &&
 		echo "content 1 txt 23 bytes" >file1.txt &&
 		p4 add file1.txt &&
 		echo "content 2-3 bin 25 bytes" >file2.dat &&