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

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>