make config --add behave correctly for empty and NULL values
Currently if we have a config file like,
[foo]
baz
bar =
and we try something like, "git config --add foo.baz roll", Git will
segfault. Moreover, for "git config --add foo.bar roll", it will
overwrite the original value instead of appending after the existing
empty value.
The problem lies with the regexp used for simulating --add in
`git_config_set_multivar_in_file()`, "^$", which in ideal case should
not match with any string but is true for empty strings. Instead use a
regexp like "a^" which can not be true for any string, empty or not.
For removing the segfault add a check for NULL values in `matches()` in
config.c.
Signed-off-by: Tanay Abhra <tanayabh@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>