Documentation/git-svn: Promote the use of --prefix in docs + examples

Documentation/git-svn: Promote the use of --prefix in docs + examples

Currently, the git-svn defaults to using an empty prefix, which ends
up placing the SVN-tracking refs directly in refs/remotes/*. This
placement runs counter to Git's convention of placing remote-tracking
branches in refs/remotes/$remote/*.

Furthermore, combining git-svn with "regular" Git remotes run the risk
of clobbering refs under refs/remotes (e.g. if you have a git remote
called "tags" with a "v1" branch, it will overlap with the git-svn's
tracking branch for the "v1" tag from Subversion.

Even though the git-svn refs stored in refs/remotes/* are not "proper"
remote-tracking branches (since they are not covered by a proper git
remote's refspec), they clearly represent a similar concept, and would
benefit from following the same convention.

For example, if git-svn tracks Subversion branch "foo" at
refs/remotes/foo, and you create a local branch refs/heads/foo to add
some commits to be pushed back to Subversion (using "git svn dcommit),
then it is clearly unhelpful of Git to throw

  warning: refname 'foo' is ambiguous.

every time you checkout, rebase, or otherwise interact with the branch.

At this time, the user is better off using the --prefix=foo/ (the
trailing slash is important) to git svn init/clone, to cause the
SVN-tracking refs to be placed at refs/remotes/foo/* instead of
refs/remotes/*. This patch updates the documentation to encourage
use of --prefix.

This is also in preparation for changing the default value of --prefix
at some point in the future.

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