checkpatch: cache last camelcase hash as .checkpatch-camelcase.<commit>

Add a file to cache the CamelCase variables found by <commit> to reduce
the time it takes to scan the include/ directory.

Filename is '.checkpatch-camelcase.<commit>' and it is created only only
if a .git directory exists.

<commit> is determined by the last non-merge commit id in the
include/ path.

Reduces checkpatch run time by ~12 cpu seconds on my little netbook.

Signed-off-by: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>