git-svn: add support for metadata in .git/config

Of course, we handle metadata migrations from previous versions
and we have added unit tests.

The new .git/config remotes resemble non-SVN remotes.  Below
is an example with comments:

[svn-remote "git-svn"]
	; like non-svn remotes, we have one URL per-remote
	url = http://foo.bar.org/svn

	; 'fetch' keys are done in the same way as non-svn
	; remotes, too.  With the left-hand-side of the ':'
	; being the remote (SVN) repository path relative to the
	; above 'url' key; and the right-hand-side being a
	; remote ref in git (refs/remotes/*).
	; An empty left-hand-side means that it will fetch
	; the entire contents of the 'url' key.
	; old-style (migrated from previous versions of git-svn)
	; are like this:
	fetch = :refs/remotes/git-svn

	; this is created by a current version of git-svn
	; using the multi-init command with an explicit
	; url (specified above).  This allows multi-init
	; to reuse SVN::Ra connections.
	fetch = trunk:refs/remotes/trunk
	fetch = branches/a:refs/remotes/a
	fetch = branches/b:refs/remotes/b
	fetch = tags/0.1:refs/remotes/tags/0.1
	fetch = tags/0.2:refs/remotes/tags/0.2
	fetch = tags/0.3:refs/remotes/tags/0.3

[svn-remote "alt"]
	; this is another old-style remote migrated over
	; to the new config format
	url = http://foo.bar.org/alt
	fetch = :refs/remotes/alt

Signed-off-by: Eric Wong <normalperson@yhbt.net>