output: print filename under search if there are many

when searching over multiple files, it's unclear where a type is coming
from. zdoc now outputs the filename before printing out search results.

src/main.zig

@@ -63,6 +63,10 @@ pub fn main() !void {
     };
     const list = try expandSourcePath(alloc, zsource);
     for (list.items) |src| {
+        // no need to print the filename under search if there's only one.
+        if (list.items.len > 1) {
+            ais.setSearchFile(src);
+        }
         // todo: consider replacing arena with something else to dealloc already
         // analyzed files early and possibly reduce memory footprint.
         const contents = try readFile(alloc, src);

src/output.zig

@@ -2651,11 +2651,30 @@ fn AutoIndentingStream(comptime UnderlyingWriter: type) type {
         /// not used until the next line
         indent_next_line: usize = 0,
 
+        /// indicates whether at least one search_file has ever been printed out.
+        first_search_file: bool = true,
+        /// the name of the file currently under search.
+        /// the ield is reset to null once printed out.
+        /// use setSearchFile to set a new name before starting a search.
+        search_file: ?[]const u8 = null,
+
         pub fn writer(self: *Self) Writer {
             return .{ .context = self };
         }
 
         pub fn write(self: *Self, bytes: []const u8) WriteError!usize {
+            if (self.search_file) |name| {
+                if (!self.first_search_file) {
+                    try self.insertNewline();
+                }
+                const n = @minimum(name.len, 60);
+                const prefix = if (name.len > n) "..." else "";
+                const truncname = name[name.len - n .. name.len];
+                try std.fmt.format(self.underlying_writer, "//--- file: {s}{s} ---\n", .{ prefix, truncname });
+                self.search_file = null;
+                self.first_search_file = false;
+            }
+
             if (bytes.len == 0)
                 return @as(usize, 0);
 
@@ -2663,6 +2682,13 @@ fn AutoIndentingStream(comptime UnderlyingWriter: type) type {
             return self.writeNoIndent(bytes);
         }
 
+        /// setSearchFile records the name to print out before the first self.write
+        /// occurs. the name is always printed in between newlines.
+        pub fn setSearchFile(self: *Self, name: []const u8) void {
+            self.search_file = name;
+            self.resetLine();
+        }
+
         // Change the indent delta without changing the final indentation level
         pub fn setIndentDelta(self: *Self, new_indent_delta: usize) void {
             if (self.indent_delta == new_indent_delta) {