scripts: let checkpatch.pl process an entire GIT branch

Currently before submitting a series, devs should run checkpatch.pl
across each patch to be submitted. This can be automated using a
command such as:

  git rebase -i master -x 'git show | ./scripts/checkpatch.pl -'

This is rather long winded to type, so this patch introduces a way
to tell checkpatch.pl to validate a series of GIT revisions.

There are now three modes it can operate in 1) check a patch 2) check a source
file, or 3) check a git branch.

If no flags are given, the mode is determined by checking the args passed to
the command. If the args contain a literal ".." it is treated as a GIT revision
list. If the args end in ".patch" or equal "-" it is treated as a patch file.
Otherwise it is treated as a source file.

This automatic guessing can be overridden using --[no-]patch --[no-]file or
--[no-]branch

For example to check a GIT revision list:

    $ ./scripts/checkpatch.pl master..
    total: 0 errors, 0 warnings, 297 lines checked

    b886d352a2bf58f0996471fb3991a138373a2957 has no obvious style problems and is ready for submission.
    total: 0 errors, 0 warnings, 182 lines checked

    2a731f9a9ce145e0e0df6d42dd2a3ce4dfc543fa has no obvious style problems and is ready for submission.
    total: 0 errors, 0 warnings, 102 lines checked

    11844169bcc0c8ed4449eb3744a69877ed329dd7 has no obvious style problems and is ready for submission.

If a genuine patch filename contains the characters '..' it is
possible to force interpretation of the arg as a patch

  $ ./scripts/checkpatch.pl --patch master..

will force it to load a patch file called "master..", or equivalently

  $ ./scripts/checkpatch.pl --no-branch master..

will simply turn off guessing of GIT revision lists.

Signed-off-by: Daniel P. Berrange <berrange@redhat.com>
Message-Id: <20170913091000.9005-1-berrange@redhat.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
