environment: remove the global variable 'merge_log_config'

environment: remove the global variable 'merge_log_config'

The global variable 'merge_log_config', set via the "merge.log" or
"merge.summary" settings, is only used in 'cmd_fmt_merge_msg()' and
'cmd_merge()' to adjust the 'shortlog_len' variable.

Remove 'merge_log_config' globally and localize it in
'cmd_fmt_merge_msg()' and 'cmd_merge()'. Set its value by passing it in
'fmt_merge_msg_config()' by passing its pointer to the function via the
callback parameter.

This change is part of an ongoing effort to eliminate global variables,
improve modularity and help libify the codebase.

Mentored-by: Christian Couder <christian.couder@gmail.com>
Mentored-by: Ghanshyam Thakkar <shyamthakkar001@gmail.com>
Signed-off-by: Ayush Chandekar <ayu.chandekar@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>