feat(installer): use go-git to clone

Integrate `go-git` into the application, and use it to `git clone`
cheatsheets when the installer runs.

Previously, the installer required that `git` be installed on the system
`PATH`, so this change has to big advantages:

1. It removes that system dependency on `git`
2. It paves the way for implementing the `--update` command

Additionally, `cheat` now performs a `--depth=1` clone when installing
cheatsheets, which should at least somewhat improve installation times
(especially on slow network connections).

vendor/github.com/ProtonMail/go-crypto/openpgp/packet/aead_config.go

@@ -0,0 +1,56 @@
+// Copyright (C) 2019 ProtonTech AG
+
+package packet
+
+import "math/bits"
+
+// AEADConfig collects a number of AEAD parameters along with sensible defaults.
+// A nil AEADConfig is valid and results in all default values.
+type AEADConfig struct {
+	// The AEAD mode of operation.
+	DefaultMode AEADMode
+	// Amount of octets in each chunk of data
+	ChunkSize uint64
+}
+
+// Mode returns the AEAD mode of operation.
+func (conf *AEADConfig) Mode() AEADMode {
+	if conf == nil || conf.DefaultMode == 0 {
+		return AEADModeEAX
+	}
+	mode := conf.DefaultMode
+	if mode != AEADModeEAX && mode != AEADModeOCB &&
+		mode != AEADModeExperimentalGCM {
+		panic("AEAD mode unsupported")
+	}
+	return mode
+}
+
+// ChunkSizeByte returns the byte indicating the chunk size. The effective
+// chunk size is computed with the formula uint64(1) << (chunkSizeByte + 6)
+func (conf *AEADConfig) ChunkSizeByte() byte {
+	if conf == nil || conf.ChunkSize == 0 {
+		return 12 // 1 << (12 + 6) == 262144 bytes
+	}
+
+	chunkSize := conf.ChunkSize
+	exponent := bits.Len64(chunkSize) - 1
+	switch {
+	case exponent < 6:
+		exponent = 6
+	case exponent > 27:
+		exponent = 27
+	}
+
+	return byte(exponent - 6)
+}
+
+// decodeAEADChunkSize returns the effective chunk size. In 32-bit systems, the
+// maximum returned value is 1 << 30.
+func decodeAEADChunkSize(c byte) int {
+	size := uint64(1 << (c + 6))
+	if size != uint64(int(size)) {
+		return 1 << 30
+	}
+	return int(size)
+}