checkpatch: improve handling of multiple patches or files

checkpatch: improve handling of multiple patches or files

Similar to how patchew output looks like for multiple patches,
say what file or patch is being tested _before_ emitting errors.
This is clearer to a human that scans the output from top to
bottom.

In addition, provide a truncated commit hash and subject instead of
the full hash, and process the commits first-to-last rather than
last-to-first.

Inspired by Linux commit 0dea9f1eef86bedacad91b6f652ca1ab0d08854c
("checkpatch: reduce number of `git log` calls with --git", 2016-03-20).

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>