bug#9993: `vc-svn-checkin' needs "--non-interactive"

[Richard Copley <rcopley@gmail.com>]

When visiting a file in a Subversion working copy, if the repository
url is http and the server requires authentication for check-ins, when
you type "C-x v v" to check in (the file has local changes), Emacs
hangs (presumably svn thinks it has prompted for credentials and is
awaiting a response). To recover, it is necessary to kill the
"svn.exe" process and run "svn cleanup" on the (now locked) working
copy.

`vc-svn-checkin' should pass "--non-interactive" to the svn
executable. (Checking-in still won't work, but at least you get an
error promptly.)  As a workaround, set the variable
`vc-svn-checkin-switches' to "--non-interactive".

All svn invocations are probably potentially affected, depending on
how the server is set up. Indeed, I don't see why vc-mode shouldn't
pass "--non-interactive" to every svn invocation.

In GNU Emacs 24.0.91.1 (i386-mingw-nt5.1.2600)
 of 2011-10-31 on MARVIN
Windowing system distributor `Microsoft Corp.', version 5.1.2600
configured using `configure --with-gcc (4.6) --no-opt --cflags
-I"D:/devel/emacs/libs/libXpm-3.5.8/include"
-I"D:/devel/emacs/libs/libXpm-3.5.8/src"
-I"D:/devel/emacs/libs/libpng-dev_1.4.3-1/include"
-I"D:/devel/emacs/libs/zlib-dev_1.2.5-2/include"
-I"D:/devel/emacs/libs/giflib-4.1.4-1/include"
-I"D:/devel/emacs/libs/jpeg-6b-4/include"
-I"D:/devel/emacs/libs/tiff-3.8.2-1/include"
-I"D:/devel/emacs/libs/gnutls-2.10.1/include" --ldflags
-L"D:/devel/emacs/libs/gnutls-2.10.1/lib"'

Important settings:
  value of $LC_ALL: nil
  value of $LC_COLLATE: nil
  value of $LC_CTYPE: nil
  value of $LC_MESSAGES: nil
  value of $LC_MONETARY: nil
  value of $LC_NUMERIC: nil
  value of $LC_TIME: nil
  value of $LANG: ENG
  value of $XMODIFIERS: nil
  locale-coding-system: cp1252
  default enable-multibyte-characters: t

Major mode: Help

Minor modes in effect:
  tooltip-mode: t
  mouse-wheel-mode: t
  tool-bar-mode: t
  menu-bar-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  font-lock-mode: t
  blink-cursor-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  line-number-mode: t
  transient-mark-mode: t