Add a debug tool to find a dependency chain

Add a new command ninja -t path <root> <dep>, to find
a dependency chain in which root depends on dep.

This is implemented as a bfs from dep.

Bug: 118089975
Test: ./ninja -t path ninja build/graph.o
      ninja
      build/libninja.a
      build/graph.o

Change-Id: I7ea230239a242d374e47957a9b1df136745dc71e
1 file changed