refactor/diff: centralize Git extended-header structure (#96)

Parsing empty Git diffs and reversing them must agree on which headers and directional pairs carry meaning. A single internal parsed view now owns that vocabulary while retaining raw lines, so future changes cannot make those paths drift apart.

bump minimum go 1.14 -> 1.20

Previous commit adding reverse diff resulted in test failures due to
using newer stdlib features.

include full license from python-unidiff

Merge pull request #55 from sourcegraph/mrnugget/fix-non-extended-fil…

…e-header-parsing

Fix parsing of non-extended file headers and lines starting with --

remove protobuf types (#51)

They are unused, so we can keep the generated output. Additionally this remove
a dependency on pbtypes.Timestamp

Add basic support for binary diffs (#46)

Add support for special characters in file names (#45)

Merge pull request #32 from sourcegraph/update-readme-import-path

update import path in README

Merge pull request #22 from dmitris/gomod

add go.mod files, set GO111MODULE=on in Travis build