Use git-format-patch to generate patch

pre-upload.py feeds patches of commits to checkpatch.pl for style check.
However it generates patches using git-show instead of git-format-patch.
The problem with git-show is that its output format is configurable
through git-config, and so it might generate a illegitimate formatted
patch.  Besides, git-format-patch should be used to generate patches
anyway.

BUG=none
TEST=Run commands below:
       cd path/to/kernel
       git config --local --add format.pretty oneline
       path/to/pre-upload.py
     You should see no "ERROR: Missing Signed-off-by: line(s)".

Change-Id: Ica7e2f91ba67a4ba4d4ba1034c9594c67126b06d
Reviewed-on: https://gerrit.chromium.org/gerrit/46347
Tested-by: Che-Liang Chiou <clchiou@chromium.org>
Reviewed-by: Mandeep Singh Baines <msb@chromium.org>
Reviewed-by: Ryan Cui <rcui@chromium.org>
Commit-Queue: Che-Liang Chiou <clchiou@chromium.org>
1 file changed