Fix inifinite loop

In setup_git_directory_gently(), there is a hack to allow for stopping
the search at the drive letter.  Only that the patch was incomplete;
we really have to stop there, instead of looping infinitely.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

setup.c

@@ -260,7 +260,7 @@ const char *setup_git_directory_gently(int *nongit_ok)
 		for (;;) {
 			if (is_git_directory(".git"))
 				break;
-			if (offset == 0) {
+			if (offset == minoffset) {
 				offset = -1;
 				break;
 			}