checkpatch: allow not using -f with files that are in git

checkpatch: allow not using -f with files that are in git

If a file exists in git and checkpatch is used without the -f flag for
scanning a file, then checkpatch will scan the file assuming it's a patch
and emit:

ERROR: Does not appear to be a unified-diff format patch

Change the behavior to assume the -f flag if the file exists in git.

[joe@perches.com: fix git "fatal" warning if file argument outside kernel tree]
Link: https://lkml.kernel.org/r/b6afa04112d450c2fc120a308d706acd60cee294.camel@perches.com
Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Julia Lawall <julia.lawall@inria.fr>
Cc: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Link: https://lkml.kernel.org/r/45b81a48e1568bd0126a96f5046eb7aaae9b83c9.camel@perches.com
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>