config: arbitrary number of matches for --unset and --replace-all

config: arbitrary number of matches for --unset and --replace-all

git-config used a static match array to hold the matches we want to
unset/replace when using --unset or --replace-all.  Use a
variable-sized array instead.

This in particular fixes the symptoms git-svn had when storing large
numbers of svn-remote.*.added-placeholder entries in the config file.

While the tests are rather more paranoid than just --unset and
--replace-all, the other operations already worked.  Indeed git-svn's
usage only breaks the first time *after* creating so many entries,
when it wants to unset and re-add them all.

Reported-by: Jess Hottenstein <jess.hottenstein@gmail.com>
Signed-off-by: Thomas Rast <tr@thomasrast.ch>
Signed-off-by: Junio C Hamano <gitster@pobox.com>