path: length validation respecting core.longpaths

Teach `git_path_is_valid` to respect `core.longpaths`.  Add helper
methods to validate length and set the error message appropriately.

src/path.c

@@ -285,6 +285,28 @@ GIT_INLINE(unsigned int) dotgit_flags(
 	return flags;
 }
 
+GIT_INLINE(unsigned int) length_flags(
+	git_repository *repo,
+	unsigned int flags)
+{
+#ifdef GIT_WIN32
+	int allow = 0;
+
+	if (repo &&
+	    git_repository__configmap_lookup(&allow, repo, GIT_CONFIGMAP_LONGPATHS) < 0)
+		allow = 0;
+
+	if (allow)
+		flags &= ~GIT_FS_PATH_REJECT_LONG_PATHS;
+
+#else
+	GIT_UNUSED(repo);
+	flags &= ~GIT_FS_PATH_REJECT_LONG_PATHS;
+#endif
+
+	return flags;
+}
+
 bool git_path_str_is_valid(
 	git_repository *repo,
 	const git_str *path,
@@ -297,6 +319,10 @@ bool git_path_str_is_valid(
 	if ((flags & GIT_PATH_REJECT_DOT_GIT))
 		flags = dotgit_flags(repo, flags);
 
+	/* Update the length checks based on platform */
+	if ((flags & GIT_FS_PATH_REJECT_LONG_PATHS))
+		flags = length_flags(repo, flags);
+
 	data.repo = repo;
 	data.file_mode = file_mode;
 	data.flags = flags;

src/path.h

@@ -41,4 +41,28 @@ GIT_INLINE(bool) git_path_is_valid(
 	return git_path_str_is_valid(repo, &str, file_mode, flags);
 }
 
+GIT_INLINE(int) git_path_validate_str_length(
+	git_repository *repo,
+	const git_str *path)
+{
+	if (!git_path_str_is_valid(repo, path, 0, GIT_FS_PATH_REJECT_LONG_PATHS)) {
+		if (path->size == SIZE_MAX)
+			git_error_set(GIT_ERROR_FILESYSTEM, "path too long: '%s'", path->ptr);
+		else
+			git_error_set(GIT_ERROR_FILESYSTEM, "path too long: '%.*s'", (int)path->size, path->ptr);
+
+		return -1;
+	}
+
+	return 0;
+}
+
+GIT_INLINE(int) git_path_validate_length(
+	git_repository *repo,
+	const char *path)
+{
+	git_str str = GIT_STR_INIT_CONST(path, SIZE_MAX);
+	return git_path_validate_str_length(repo, &str);
+}
+
 #endif