cmd/dist: reflect local changes to tree in goversion

runtime.Version() requires a trailing "+" when
tree had local modifications at time of build.

Fixes #7701

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/84040045

src/cmd/dist/build.c

@@ -242,12 +242,13 @@ findgoversion(void)
 {
 	char *tag, *rev, *p;
 	int i, nrev;
-	Buf b, path, bmore, branch;
+	Buf b, path, bmore, bplus, branch;
 	Vec tags;
 
 	binit(&b);
 	binit(&path);
 	binit(&bmore);
+	binit(&bplus);
 	binit(&branch);
 	vinit(&tags);
 
@@ -314,11 +315,16 @@ findgoversion(void)
 		// Add extra information.
 		run(&bmore, goroot, CheckExit, "hg", "log", "--template", " +{node|short} {date|date}", "-r", rev, nil);
 		chomp(&bmore);
+		// Generate a list of local modifications, if any.
+		run(&bplus, goroot, CheckExit, "hg", "status", "-m", "-a", "-r", "-d", nil);
+		chomp(&bplus);
 	}
 
 	bprintf(&b, "%s", tag);
 	if(bmore.len > 0)
 		bwriteb(&b, &bmore);
+	if(bplus.len > 0)
+		bwritestr(&b, " +");
 
 	// Cache version.
 	writefile(&b, bstr(&path), 0);
@@ -330,6 +336,7 @@ done:
 	bfree(&b);
 	bfree(&path);
 	bfree(&bmore);
+	bfree(&bplus);
 	bfree(&branch);
 	vfree(&tags);