path: also guard `.gitmodules` against NTFS Alternate Data Streams

We just safe-guarded `.git` against NTFS Alternate Data Stream-related
attack vectors, and now it is time to do the same for `.gitmodules`.

Note: In the added regression test, we refrain from verifying all kinds
of variations between short names and NTFS Alternate Data Streams: as
the new code disallows _all_ Alternate Data Streams of `.gitmodules`, it
is enough to test one in order to know that all of them are guarded
against.

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

src/path.c

@@ -1646,7 +1646,7 @@ GIT_INLINE(bool) only_spaces_and_dots(const char *path)
 	const char *c = path;
 
 	for (;; c++) {
-		if (*c == '\0')
+		if (*c == '\0' || *c == ':')
 			return true;
 		if (*c != ' ' && *c != '.')
 			return false;

tests/path/dotgit.c

@@ -116,4 +116,5 @@ void test_path_dotgit__dotgit_modules_symlink(void)
 	cl_assert_equal_b(true, git_path_isvalid(NULL, ".gitmodules", 0, GIT_PATH_REJECT_DOT_GIT_HFS|GIT_PATH_REJECT_DOT_GIT_NTFS));
 	cl_assert_equal_b(false, git_path_isvalid(NULL, ".gitmodules", S_IFLNK, GIT_PATH_REJECT_DOT_GIT_HFS));
 	cl_assert_equal_b(false, git_path_isvalid(NULL, ".gitmodules", S_IFLNK, GIT_PATH_REJECT_DOT_GIT_NTFS));
+	cl_assert_equal_b(false, git_path_isvalid(NULL, ".gitmodules . .::$DATA", S_IFLNK, GIT_PATH_REJECT_DOT_GIT_NTFS));
 }