Merge branch 'rr/submodule-diff-config'

Allow "git diff --submodule=log" to set to be the default via
configuration.

* rr/submodule-diff-config:
submodule: display summary header in bold
diff: rename "set" variable
diff: introduce diff.submodule configuration variable
Documentation: move diff.wordRegex from config.txt to diff-config.txt

diff --cc diff.c
Simple merge