mailmap: drop global config variables

mailmap: drop global config variables

The 'mailmap.file' and 'mailmap.blob' configurations are currently
parsed and stored in the global variables 'git_mailmap_file' and
'git_mailmap_blob'. Since these values are typically only needed once
when initializing a mailmap, there is no need to keep them as global
state throughout the lifetime of the Git process.

To reduce global state, remove these global variables and instead use
'repo_config_get_*' functions to read the configuration on demand.

Signed-off-by: Burak Kaan Karaçay <bkkaracay@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>