cmd/go: fix module replace flag parsed bug

In CL 122404, we change -replace syntax from => to =.
And we also need to change this and the tests.

Fixes golang/go#26373.

Change-Id: I2d4e85e10c1578540cc7673b93d849270940d776
Reviewed-on: https://go-review.googlesource.com/123778Reviewed-by: Bryan C. Mills <bcmills@google.com>
Run-TryBot: Bryan C. Mills <bcmills@google.com>

src/cmd/go/internal/modcmd/mod.go

@@ -400,7 +400,7 @@ func flagReplace(arg string) {
 	if i = strings.Index(arg, "="); i < 0 {
 		base.Fatalf("go mod: -replace=%s: need old@v=new[@v] (missing =)", arg)
 	}
-	old, new := strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+2:])
+	old, new := strings.TrimSpace(arg[:i]), strings.TrimSpace(arg[i+1:])
 	if strings.HasPrefix(new, ">") {
 		base.Fatalf("go mod: -replace=%s: separator between old and new is =, not =>", arg)
 	}

src/cmd/go/mod_test.go

@@ -383,8 +383,8 @@ func TestModEdit(t *testing.T) {
 		"-droprequire=x.2",
 		"-exclude=x.1 @ v1.2.0",
 		"-exclude=x.1@v1.2.1",
-		"-replace=x.1@v1.3.0=>y.1@v1.4.0",
-		"-replace=x.1@v1.4.0 => ../z",
+		"-replace=x.1@v1.3.0=y.1@v1.4.0",
+		"-replace=x.1@v1.4.0 = ../z",
 	)
 	mustHaveGoMod(`module x.x/y/z
 
@@ -451,8 +451,8 @@ require x.3 v1.99.0
 	}
 
 	tg.run("mod",
-		"-replace=x.1@v1.3.0=>y.1/v2@v2.3.5",
-		"-replace=x.1@v1.4.0=>y.1/v2@v2.3.5",
+		"-replace=x.1@v1.3.0=y.1/v2@v2.3.5",
+		"-replace=x.1@v1.4.0=y.1/v2@v2.3.5",
 	)
 	mustHaveGoMod(`module x.x/y/z
 
@@ -466,7 +466,7 @@ replace (
 require x.3 v1.99.0
 `)
 	tg.run("mod",
-		"-replace=x.1=>y.1/v2@v2.3.6",
+		"-replace=x.1=y.1/v2@v2.3.6",
 	)
 	mustHaveGoMod(`module x.x/y/z