Add a field prefix to the deprecated log statements

To make theme easier to spot, esp. on INFO level.

common/hugo/hugo.go

@@ -369,7 +369,7 @@ func DeprecateLevel(item, alternative, version string, level logg.Level) {
 		msg = fmt.Sprintf("%s was deprecated in Hugo %s and will be removed in a future release. %s", item, version, alternative)
 	}
 
-	loggers.Log().Logger().WithLevel(level).Logf(msg)
+	loggers.Log().Logger().WithLevel(level).WithField(loggers.FieldNameCmd, "deprecated").Logf(msg)
 }
 
 // We ususally do about one minor version a month.

testscripts/commands/deprecate.txt

@@ -1,13 +1,13 @@
 
+# Test deprecation logging.
+hugo -e info --logLevel info
+stdout 'INFO  deprecated: item was deprecated in Hugo'
 
-hugo -e info  --logLevel info
-stdout 'INFO  item was deprecated in Hugo'
+hugo -e warn --logLevel warn
+stdout 'WARN  deprecated: item was deprecated in Hugo'
 
-hugo -e warn  --logLevel warn
-stdout 'WARN  item was deprecated in Hugo'
-
-! hugo -e error  --logLevel warn
-stdout 'ERROR item was deprecated in Hugo'
+! hugo -e error --logLevel warn
+stdout 'ERROR deprecated: item was deprecated in Hugo'
 
 -- hugo.toml --
 baseURL = "https://example.com/"