os: improve Windows fixLongPath

CL 574695 added caching the os.Chdir argument for Windows, and used the
cached value to assess the length of the current working directory in
addExtendedPrefix (used by fixLongPath).

It did not take into account that Chdir can accept relative paths, and
thus the pathLength calculation in addExtendedPrefix can be wrong.

Let's only cache the os.Chdir argument if it's absolute, and clean the
cache otherwise, thus improving the correctness of fixLongPath.

For golang#41734
For golang#21782
For golang#36375

Change-Id: Ie24a5ed763a7aacc310666d2e4cbb8e298768670
Reviewed-on: https://go-review.googlesource.com/c/go/+/607437
Reviewed-by: Ian Lance Taylor <[email protected]>
Reviewed-by: Alex Brainman <[email protected]>
Reviewed-by: Damien Neil <[email protected]>
Auto-Submit: Ian Lance Taylor <[email protected]>
Reviewed-by: Quim Muntal <[email protected]>
Reviewed-by: Cherry Mui <[email protected]>
LUCI-TryBot-Result: Go LUCI <[email protected]>

Author: kolyshkin

src/os/file.go

@@ -344,8 +344,13 @@ func Chdir(dir string) error {
 		return &PathError{Op: "chdir", Path: dir, Err: e}
 	}
 	if runtime.GOOS == "windows" {
+		abs := filepathlite.IsAbs(dir)
 		getwdCache.Lock()
-		getwdCache.dir = dir
+		if abs {
+			getwdCache.dir = dir
+		} else {
+			getwdCache.dir = ""
+		}
 		getwdCache.Unlock()
 	}
 	if log := testlog.Logger(); log != nil {