completion: __git_config_get_set_variables() handle values with spaces

completion: __git_config_get_set_variables() handle values with spaces

Commit 0065236 (bash completion: complete variable names for "git
config" with options 2009-05-08) implemented its config variable search
wrong. When a config contains a value with a space and a period (.) in
it, completion erroneously thinks that line in the configuration is
multiple config variables.

For example

 $ cat .git/config
   format.cc = Junio C Hamano <gitster@pobox.com>

 $ git config --unset <TAB>
   format.cc
   <gitster@pobox.com>

Instead of using a for loop splitting across spaces, pipe each line to a
while read loop and beef up the case statement to match only
'config.variable=value'.

Signed-off-by: Stephen Boyd <bebarino@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>