range-diff: show submodule changes irrespective of diff.submodule

range-diff: show submodule changes irrespective of diff.submodule

After generating diffs for each range to be compared using a 'git log'
invocation, range-diff.c::read_patches looks for the "diff --git" header
in those diffs to recognize the beginning of a new change.

In a project with submodules, and with 'diff.submodule=log' set in the
config, this header is missing for the diff of a changed submodule, so
any submodule changes are quietly ignored in the range-diff.

When 'diff.submodule=diff' is set in the config, the "diff --git" header
is also missing for the submodule itself, but is shown for submodule
content changes, which can easily confuse 'git range-diff' and lead to
errors such as:

    error: git apply: bad git-diff - inconsistent old filename on line 1
    error: could not parse git header 'diff --git path/to/submodule/and/some/file/within
    '
    error: could not parse log for '@{u}..@{1}'

Force the submodule diff format to its default ("short") when invoking
'git log' to generate the patches for each range, such that submodule
changes are always detected.

Add a test, including an invocation with '--creation-factor=100' to
force the second commit in the range not to be considered a complete
rewrite, in order to verify we do indeed get the "short" format.

Signed-off-by: Philippe Blain <levraiphilippeblain@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>