completion: fix issue with process substitution not working on Git for Windows

Git for Windows comes with a bash that doesn't support process substitution.
It issues the following error when using git-completion.bash with
GIT_PS1_SHOWUPSTREAM set:

$ export GIT_PS1_SHOWUPSTREAM=1
sh.exe": cannot make pipe for process substitution: Function not implemented
sh.exe": cannot make pipe for process substitution: Function not implemented
sh.exe": <(git config -z --get-regexp '^(svn-remote\..*\.url|bash\.showupstream)$' 2>/dev/null | tr '\0\n' '\n '): ambiguous redirect

Replace the process substitution with a 'here string'.

Signed-off-by: Stefan Naewe <stefan.naewe@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-completion.bash

@@ -110,6 +110,7 @@ __git_ps1_show_upstream ()
 	local upstream=git legacy="" verbose=""
 
 	# get some config options from git-config
+	output="$(git config -z --get-regexp '^(svn-remote\..*\.url|bash\.showupstream)$' 2>/dev/null | tr '\0\n' '\n ')"
 	while read key value; do
 		case "$key" in
 		bash.showupstream)
@@ -125,7 +126,7 @@ __git_ps1_show_upstream ()
 			upstream=svn+git # default upstream is SVN if available, else git
 			;;
 		esac
-	done < <(git config -z --get-regexp '^(svn-remote\..*\.url|bash\.showupstream)$' 2>/dev/null | tr '\0\n' '\n ')
+	done <<< "$output"
 
 	# parse configuration values
 	for option in ${GIT_PS1_SHOWUPSTREAM}; do