remote-helpers: add testgit helper

Currently the remote helper infrastructure is only used by the curl
helper, which does not give a good impression of how remote helpers
can be used to interact with foreign repositories. Since implementing
such a helper is non-trivial it would be good to have at least one
easy-to-follow example demonstrating how to implement a helper that
interacts with a foreign vcs using fast-import/fast-export.

The testgit helper can be used to interact with remote git
repositories by prefixing the url with "testgit::".

Signed-off-by: Junio C Hamano <gitster@pobox.com>

git_remote_helpers/git/exporter.py

@@ -0,0 +1,51 @@
+import os
+import subprocess
+import sys
+
+
+class GitExporter(object):
+    """An exporter for testgit repositories.
+
+    The exporter simply delegates to git fast-export.
+    """
+
+    def __init__(self, repo):
+        """Creates a new exporter for the specified repo.
+        """
+
+        self.repo = repo
+
+    def export_repo(self, base):
+        """Exports a fast-export stream for the given directory.
+
+        Simply delegates to git fast-epxort and pipes it through sed
+        to make the refs show up under the prefix rather than the
+        default refs/heads. This is to demonstrate how the export
+        data can be stored under it's own ref (using the refspec
+        capability).
+        """
+
+        dirname = self.repo.get_base_path(base)
+        path = os.path.abspath(os.path.join(dirname, 'testgit.marks'))
+
+        if not os.path.exists(dirname):
+            os.makedirs(dirname)
+
+        print "feature relative-marks"
+        if os.path.exists(os.path.join(dirname, 'git.marks')):
+            print "feature import-marks=%s/git.marks" % self.repo.hash
+        print "feature export-marks=%s/git.marks" % self.repo.hash
+        sys.stdout.flush()
+
+        args = ["git", "--git-dir=" + self.repo.gitpath, "fast-export", "--export-marks=" + path]
+
+        if os.path.exists(path):
+            args.append("--import-marks=" + path)
+
+        args.append("HEAD")
+
+        p1 = subprocess.Popen(args, stdout=subprocess.PIPE)
+
+        args = ["sed", "s_refs/heads/_" + self.repo.prefix + "_g"]
+
+        subprocess.check_call(args, stdin=p1.stdout)