internal/span: add a filename only print for spans

This prints the base of the path rather than the whole path when printing a
span.
This is useful for places where you are printing for the user rather than for
machines.
The format is %f

Change-Id: I6d1a52e4583099ff298c1fb645272578a49472eb
Reviewed-on: https://go-review.googlesource.com/c/tools/+/174942
Run-TryBot: Ian Cottrell <iancottrell@google.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Rebecca Stambler <rstambler@golang.org>

internal/span/span.go

@@ -7,6 +7,7 @@ package span
 import (
 	"encoding/json"
 	"fmt"
+	"path"
 )
 
 // Span represents a source code range in standardized form.
@@ -167,7 +168,9 @@ func (s Span) Format(f fmt.State, c rune) {
 	// we should always have a uri, simplify if it is file format
 	//TODO: make sure the end of the uri is unambiguous
 	uri := string(s.v.URI)
-	if !fullForm {
+	if c == 'f' {
+		uri = path.Base(uri)
+	} else if !fullForm {
 		if filename, err := s.v.URI.Filename(); err == nil {
 			uri = filename
 		}