git-prompt.sh: make $f local to __git_eread()

This function uses (non-local) $f to store the value of its first parameter.
This can interfere with the user's environment.

Signed-off-by: Justin Guenther <jguenther@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

contrib/completion/git-prompt.sh

@@ -261,7 +261,7 @@ __git_ps1_colorize_gitstring ()
 
 eread ()
 {
-	f="$1"
+	local f="$1"
 	shift
 	test -r "$f" && read "$@" <"$f"
 }