Dependencies between generated targets sometimes missing

[KevinJiao]

Hi again,

There's a subtle bug in the way dependencies on generated files are handled in the implementation of #82 resulting in missing impacted targets when the generating target is lexicographically before the generated target.

I have an example here (https://github.com/KevinJiao/gen-file-repro) with logs

~/projects/bazel-diff-source$ ./bazel-diff-example.sh ~projects/genrule-repro /usr/bin/bazel master^ master
Generating Hashes for Revision 'master^'
Starting local Bazel server and connecting to it...
...
Generating Hashes for Revision 'master'
...
Determining Impacted Targets
Impacted Targets between master^ and master:
//:gen_test //:gen_test.sh //:aaa_test.txt

Here I would also expect //:compare_files to be impacted, as it depends on //:aaa_test.txt

[KevinJiao]

I've opened a PR that fixes the issue #91, let me know if you'd like any changes.

[KevinJiao]

Closed by #91