Keep git's own words in a fetch's reason: a bare "error:" line no longer stands for the forge's refusal

The mirror of jschoubben/car-hunter on ace said only "git -C: error:": git writes a bare
"error:" before the forge's message, and the reason kept the first non-empty line. forgekey.Said
keeps every line that says something, leaves bare prefixes out, joins them and bounds them at
600 bytes; every git and ssh-keygen reason of buildstep and forgekey goes through it, and the
mirror's commands are named by their verb, not by -C.

go/forgekey/runner.go

@@ -9,6 +9,7 @@ import (
 	"os/exec"
 	"strings"
 	"time"
+	"unicode/utf8"
 )
 
 // Ran is what a command did: its output, its exit status, and why it never ran to an answer.
@@ -108,11 +109,37 @@ func stableDir(clean bool, env []string) string {
 	return os.TempDir()
 }
 
-func firstLine(s string) string {
+// SaidBound bounds what Said keeps of a command's words, so a reason stays a line of a step's record and of a log.
+const SaidBound = 600
+
+// Said is what a command said of its failure in its own words, for a reason: every line that says something, trimmed
+// and joined with "; ", bounded at SaidBound bytes. A line that is a bare prefix and nothing else (git writes "error:"
+// alone before and after the forge's own message, "remote:" or "fatal:" likewise) is left out, so the cause is never
+// cut off behind it: the first line alone of `git fetch` from a repository the account cannot read was "error:".
+func Said(s string) string {
+	var keep []string
 	for _, l := range strings.Split(s, "\n") {
-		if l = strings.TrimSpace(l); l != "" {
-			return l
+		l = strings.TrimSpace(l)
+		if l == "" || barePrefix(l) {
+			continue
 		}
+		keep = append(keep, l)
 	}
-	return ""
+	out := strings.Join(keep, "; ")
+	if len(out) > SaidBound {
+		cut := SaidBound
+		for cut > 0 && !utf8.RuneStart(out[cut]) {
+			cut--
+		}
+		out = out[:cut] + " …"
+	}
+	return out
+}
+
+// barePrefix says whether a line is one word and a colon, such as "error:" or "remote:", with nothing after it.
+func barePrefix(l string) bool {
+	if !strings.HasSuffix(l, ":") {
+		return false
+	}
+	return !strings.ContainsAny(strings.TrimSuffix(l, ":"), " \t:")
 }