checkpatch: fix detection of git repository

Since git v1.7.7, the .git directory can be a file when, for example,
the kernel is a submodule of another git super project.  So, the check
"-d .git" is not working anymore in this case.  Using a more generic
check like "-e .git" corrects this behaviour.

Signed-off-by: Richard Genoud <richard.genoud@gmail.com>
Cc: Andy Whitcroft <apw@canonical.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>