pull: parse pull.ff as a bool or string

Since b814da8 (pull: add pull.ff configuration, 2014-01-15) git-pull
supported setting --(no-)ff via the pull.ff configuration value.
However, as it only matches the string values of "true" and "false", it
does not support other boolean aliases such as "on", "off", "1", "0".
This is inconsistent with the merge.ff setting, which supports these
aliases.

Fix this by using the bool_or_string_config function to retrieve the
value of pull.ff.

Signed-off-by: Paul Tan <pyokagan@gmail.com>
Reviewed-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

git-pull.sh

@@ -54,7 +54,7 @@ then
 fi
 
 # Setup default fast-forward options via `pull.ff`
-pull_ff=$(git config pull.ff)
+pull_ff=$(bool_or_string_config pull.ff)
 case "$pull_ff" in
 true)
 	no_ff=--ff