maintenance: use 'git config --fixed-value'

maintenance: use 'git config --fixed-value'

When a repository's leading directories contain regex metacharacters,
the config calls for 'git maintenance register' and 'git maintenance
unregister' are not careful enough. Use the new --fixed-value option
to direct the config machinery to use exact string matches. This is a
more robust option than escaping these arguments in a piecemeal fashion.

For the test, require that we are not running on Windows since the '+'
and '*' characters are not allowed on that filesystem.

Reported-by: Emily Shaffer <redacted>
Reported-by: Jonathan Nieder <redacted>
Signed-off-by: Derrick Stolee <redacted>
Signed-off-by: Junio C Hamano <redacted>