Add the DiffContextSize setting to GitConfig

It defaults to 3 lines, which is the default value for git.

docs/Config.md

@@ -76,6 +76,7 @@ git:
   overrideGpg: false # prevents lazygit from spawning a separate process when using GPG
   disableForcePushing: false
   parseEmoji: false
+  diffContextSize: 3 # how many lines of context are shown around a change in diffs
 os:
   editCommand: '' # see 'Configuring File Editing' section
   editCommandTemplate: '{{editor}} {{filename}}'

pkg/config/user_config.go

@@ -72,6 +72,7 @@ type GitConfig struct {
 	// this shoudl really be under 'gui', not 'git'
 	ParseEmoji bool      `yaml:"parseEmoji"`
 	Log        LogConfig `yaml:"log"`
+	DiffContextSize     int                           `yaml:"diffContextSize"`
 }
 
 type PagingConfig struct {
@@ -359,6 +360,7 @@ func GetDefaultConfig() *UserConfig {
 			DisableForcePushing: false,
 			CommitPrefixes:      map[string]CommitPrefixConfig(nil),
 			ParseEmoji:          false,
+			DiffContextSize:     3,
 		},
 		Refresher: RefresherConfig{
 			RefreshInterval: 10,