Fetch should return a unique error type when ref not found

It can be useful for callers to distinguish between an error of
"couldn't find remote ref" and some other error like "network error".

Creating an explicit error type for this allows consumers to
determine the kind of error using the errors.Is and errors.As
interface added in go1.13

Author: yabberyabber

remote.go

@@ -32,6 +32,19 @@ var (
 	ErrExactSHA1NotSupported = errors.New("server does not support exact SHA1 refspec")
 )
 
+type NoMatchingRefSpecError struct {
+	refSpec config.RefSpec
+}
+
+func (e NoMatchingRefSpecError) Error() string {
+	return fmt.Sprintf("couldn't find remote ref %q", e.refSpec.Src())
+}
+
+func (e NoMatchingRefSpecError) Is(target error) bool {
+	_, ok := target.(NoMatchingRefSpecError)
+	return ok
+}
+
 const (
 	// This describes the maximum number of commits to walk when
 	// computing the haves to send to a server, for each ref in the
@@ -751,7 +764,7 @@ func doCalculateRefs(
 	})
 
 	if !matched && !s.IsWildcard() {
-		return fmt.Errorf("couldn't find remote ref %q", s.Src())
+		return NoMatchingRefSpecError{refSpec: s}
 	}
 
 	return err

remote_test.go

@@ -3,6 +3,7 @@ package git
 import (
 	"bytes"
 	"context"
+	"errors"
 	"io"
 	"io/ioutil"
 	"os"
@@ -145,6 +146,7 @@ func (s *RemoteSuite) TestFetchNonExistantReference(c *C) {
 	})
 
 	c.Assert(err, ErrorMatches, "couldn't find remote ref.*")
+	c.Assert(errors.Is(err, NoMatchingRefSpecError{}), Equals, true)
 }
 
 func (s *RemoteSuite) TestFetchContext(c *C) {