checkpatch: fix detection of git repository

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 <[email protected]>
Cc: Andy Whitcroft <[email protected]>
Cc: Joe Perches <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>