Fix --reference option under Windows

When intializing a new repo with the --reference option on Windows 10
the objects/info/alternates in each git repository is created with
Windows line endings (\r\n), leading to the following error:

error: object directory C:/<PATH_TO_MIRROR>/<REPO_NAME>.git/objects?
does not exist; check .git/objects/info/alternates

This can be fixed by simply using unix line endings on both
Windows and unix platforms.

Reported-by: Francisco Javier Alvarez Garcia <javier.alvarez.garcia.17@gmail.com>
Follow-up-from: I268fe029ede68802c21037b0f2ae8a95afb85e48
Bug: https://crbug.com/gerrit/13208
Change-Id: I6da60c4ca957778b3c42ab6b9ad85c40483f0042
Reviewed-on: https://gerrit-review.googlesource.com/c/git-repo/+/289431
Reviewed-by: Mike Frysinger <vapier@google.com>
Tested-by: Remy Bohmer <oss@bohmer.net>

project.py

@@ -62,7 +62,8 @@ RETRY_JITTER_PERCENT = 0.1
 def _lwrite(path, content):
   lock = '%s.lock' % path
 
-  with open(lock, 'w') as fd:
+  # Maintain Unix line endings on all OS's to match git behavior.
+  with open(lock, 'w', newline='\n') as fd:
     fd.write(content)
 
   try: