git-svn: Warn about changing default for --prefix in Git v2.0

In Git v2.0, we will change the default --prefix for init/clone from
none/empty to "origin/" (which causes SVN-tracking branches to be
placed at refs/remotes/origin/* instead of refs/remotes/*).

This patch warns users about the upcoming change, both in the git-svn
manual page, and on stderr when running init/clone in the "multi-mode"
without providing a --prefix.

Cc: Eric Wong <normalperson@yhbt.net>
Signed-off-by: Johan Herland <johan@herland.net>
Signed-off-by: Eric Wong <normalperson@yhbt.net>

Documentation/git-svn.txt

@@ -86,6 +86,14 @@ COMMANDS
 	(refs/remotes/$remote/*). Setting a prefix is also useful
 	if you wish to track multiple projects that share a common
 	repository.
++
+NOTE: In Git v2.0, the default prefix will CHANGE from "" (no prefix)
+to "origin/". This is done to put SVN-tracking refs at
+"refs/remotes/origin/*" instead of "refs/remotes/*", and make them
+more compatible with how Git's own remote-tracking refs are organized
+(i.e. refs/remotes/$remote/*). You can enjoy the same benefits today,
+by using the --prefix option.
+
 --ignore-paths=<regex>;;
 	When passed to 'init' or 'clone' this regular expression will
 	be preserved as a config key.  See 'fetch' for a description
@@ -993,7 +1001,8 @@ placed at refs/remotes/origin/* rather than the default refs/remotes/*.
 The former is more compatible with the layout of Git's "regular"
 remote-tracking refs (refs/remotes/$remote/*), and may potentially
 prevent similarly named SVN branches and Git remotes from clobbering
-each other.
+each other. In Git v2.0 the default prefix used (i.e. when no --prefix
+is given) will change from "" (no prefix) to "origin/".
 
 When using multiple --branches or --tags, 'git svn' does not automatically
 handle name collisions (for example, if two branches from different paths have