Merge branch 'sg/merge-options' (early part)

* 'sg/merge-options' (early part):
merge, pull: add '--(no-)log' command line option
fmt-merge-msg: add '--(no-)log' options and 'merge.log' config variable
add 'merge.stat' config variable
merge, pull: introduce '--(no-)stat' option
doc: moved merge.* config variables into separate merge-config.txt