tests: verify diff stats with renames in subdirectory
Until now, we didn't have any tests that verified that our format for
renames in subdirectories is correct. While our current behaviour is no
different than for renames that do not happen with a common prefix
shared between old and new file name, we intend to change the format to
instead match the format that upstream git uses.
Add a test case for this to document our current behaviour and to show
how the next commit will change that format.
diff: include commit message when formatting patch
When formatting a patch as email we do not include the commit's
message in the formatted patch output. Implement this and add a
test that verifies behavior.