git-svn: add support for per-[svn-remote "..."] options

git-svn: add support for per-[svn-remote "..."] options

Available options are currently:

  svn-remote.<remote>.{noMetadata,useSvmProps,followParent}

These boolean switches will override options set globally in
[svn], and even override options set on the command-line (this
should probably change in the future, however).

Note that the noMetadata and useSvmProps options conflict.  It's
both technically and logically impossible to use them together.

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