repack: refactor to avoid double-negation of update-server-info

repack: refactor to avoid double-negation of update-server-info

By default, git-repack(1) runs `update_server_info()` to generate info
required for the dumb HTTP protocol. This can be disabled via the `-n`
flag, which then sets the `no_update_server_info` flag. Further down the
code this leads to some double-negation logic, which is about to become
more confusing as we're about to add a new config which allows the user
to permanently disable generation of the info.

Refactor the code to avoid the double-negation and add some tests which
verify that the flag continues to work as expected.

Signed-off-by: Patrick Steinhardt <redacted>
Signed-off-by: Junio C Hamano <redacted>