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>