prompt: fix show upstream with svn and zsh

Currently the __git_ps1 git prompt gives the following error with a
repository converted by git-svn, when used with zsh:

   __git_ps1_show_upstream:19: bad pattern: svn_remote[
   __git_ps1_show_upstream:45: bad substitution

To reproduce the problem, the __git_ps1_show_upstream function can be
executed in a repository converted with git-svn.  Both those errors are
triggered by spaces after the '['.

Zsh also doesn't support initializing an array with `local var=(...)`.
This triggers the following error:

   __git_ps1_show_upstream:41: bad pattern: svn_upstream=(commit

Use
   local -a
   var=(...)
instead to make is compatible.

This was introduced by 6d158cba (bash completion: Support "divergence
from upstream" messages in __git_ps1), when the script was for bash
only.

Signed-off-by: Thomas Gummerer <t.gummerer@gmail.com>
Acked-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -146,7 +146,8 @@ __git_ps1_show_upstream ()
 	svn*)
 		# get the upstream from the "git-svn-id: ..." in a commit message
 		# (git-svn uses essentially the same procedure internally)
-		local svn_upstream=($(git log --first-parent -1 \
+		local -a svn_upstream
+		svn_upstream=($(git log --first-parent -1 \
 					--grep="^git-svn-id: \(${svn_url_pattern#??}\)" 2>/dev/null))
 		if [[ 0 -ne ${#svn_upstream[@]} ]]; then
 			svn_upstream=${svn_upstream[${#svn_upstream[@]} - 2]}