filter-repo: make sure git-fast-import has really finished when we exit

Also, provide an OutputStream class, to make it easy to still direct all
output to some file rather than always sending to git fast-import.

Signed-off-by: Elijah Newren <newren@gmail.com>

git-filter-repo

@@ -347,7 +347,7 @@ class FastExportFilter(object):
 def FastExportOutput(source_repo, extra_args = []):
   return Popen(["git", "fast-export", "--all", "--topo-order"] + extra_args,
                stdout = PIPE,
-               cwd = source_repo).stdout
+               cwd = source_repo)
 
 def FastImportInput(target_repo, extra_args = []):
   if not os.path.isdir(target_repo):
@@ -356,7 +356,7 @@ def FastImportInput(target_repo, extra_args = []):
       raise SystemExit("git init in %s failed!" % target_repo)
   return Popen(["git", "fast-import", "--quiet"] + extra_args,
                stdin = PIPE,
-               cwd = target_repo).stdin
+               cwd = target_repo)
 
 def get_total_commits(repo):
   p1 = Popen(["git", "rev-list", "--all"], stdout = PIPE, cwd = repo)