path: add a function to detect an .gitmodules file

Given a path component it knows what to pass to the filesystem-specific
functions so we're protected even from trees which try to use the 8.3 naming
rules to get around us matching on the filename exactly.

The logic and test strings come from the equivalent git change.

src/path.c

@@ -1840,3 +1840,16 @@ int git_path_normalize_slashes(git_buf *out, const char *path)
 
 	return 0;
 }
+
+static int verify_dotgit_generic(const char *name, const char *dotgit_name, const char *shortname_pfix)
+{
+	if (!verify_dotgit_ntfs_generic(name, dotgit_name, shortname_pfix))
+		return false;
+
+	return verify_dotgit_hfs_generic(name, strlen(name), dotgit_name, strlen(dotgit_name));
+}
+
+int git_path_is_dotgit_modules(const char *name)
+{
+	return !verify_dotgit_generic(name, "gitmodules", "gi7eba");
+}

tests/path/dotgit.c

+#include "clar_libgit2.h"
+#include "path.h"
+
+#include "git2/sys/path.h"
+
+static char *gitmodules_altnames[] = {
+	".gitmodules",
+
+	".git\u200cmodules",
+
+	".Gitmodules",
+	".gitmoduleS",
+
+	".gitmodules ",
+	".gitmodules.",
+	".gitmodules  ",
+	".gitmodules. ",
+	".gitmodules .",
+	".gitmodules..",
+	".gitmodules   ",
+	".gitmodules.  ",
+	".gitmodules . ",
+	".gitmodules  .",
+
+	".Gitmodules ",
+	".Gitmodules.",
+	".Gitmodules  ",
+	".Gitmodules. ",
+	".Gitmodules .",
+	".Gitmodules..",
+	".Gitmodules   ",
+	".Gitmodules.  ",
+	".Gitmodules . ",
+	".Gitmodules  .",
+
+	"GITMOD~1",
+	"gitmod~1",
+	"GITMOD~2",
+	"gitmod~3",
+	"GITMOD~4",
+
+	"GITMOD~1 ",
+	"gitmod~2.",
+	"GITMOD~3  ",
+	"gitmod~4. ",
+	"GITMOD~1 .",
+	"gitmod~2   ",
+	"GITMOD~3.  ",
+	"gitmod~4 . ",
+
+	"GI7EBA~1",
+	"gi7eba~9",
+
+	"GI7EB~10",
+	"GI7EB~11",
+	"GI7EB~99",
+	"GI7EB~10",
+	"GI7E~100",
+	"GI7E~101",
+	"GI7E~999",
+	"~1000000",
+	"~9999999",
+};
+
+static char *gitmodules_not_altnames[] = {
+	".gitmodules x",
+	".gitmodules .x",
+
+	" .gitmodules",
+
+	"..gitmodules",
+
+	"gitmodules",
+
+	".gitmodule",
+
+	".gitmodules x ",
+	".gitmodules .x",
+
+	"GI7EBA~",
+	"GI7EBA~0",
+	"GI7EBA~~1",
+	"GI7EBA~X",
+	"Gx7EBA~1",
+	"GI7EBX~1",
+
+	"GI7EB~1",
+	"GI7EB~01",
+	"GI7EB~1",
+};
+
+void test_path_dotgit__dotgit_modules(void)
+{
+	size_t i;
+	cl_assert_equal_i(1, git_path_is_dotgit_modules(".gitmodules"));
+	cl_assert_equal_i(1, git_path_is_dotgit_modules(".git\xe2\x80\x8cmodules"));
+
+	for (i = 0; i < ARRAY_SIZE(gitmodules_altnames); i++) {
+		const char *name = gitmodules_altnames[i];
+		if (!git_path_is_dotgit_modules(name))
+			cl_fail(name);
+	}
+
+	for (i = 0; i < ARRAY_SIZE(gitmodules_not_altnames); i++) {
+		const char *name = gitmodules_not_altnames[i];
+		if (git_path_is_dotgit_modules(name))
+			cl_fail(name);
+	}
+
+}