cmd/go/internal/vcs: avoid treating git submodule directory as repository root

Before the change, FromDir treats a directory contains a .git entry
as a git repository root, no matter whether the entry is a directory
or a normal file. When git submodule is used, the directory of the
submodule contains a .git file (it's a normal file, not a directory).
In this case, the directory of this submodule should not be treated
as the repository root.

Author: ZekeLu

src/cmd/go/internal/vcs/vcs.go

@@ -814,7 +814,12 @@ func FromDir(dir, srcRoot string, allowNesting bool) (repoDir string, vcsCmd *Cm
 	origDir := dir
 	for len(dir) > len(srcRoot) {
 		for _, vcs := range vcsList {
-			if _, err := statAny(dir, vcs.RootNames); err == nil {
+			if fi, err := statAny(dir, vcs.RootNames); err == nil {
+				// git submodule contains .git file, but its directory is not a
+				// git repository root.
+				if !fi.IsDir() && fi.Name() == ".git" {
+					continue
+				}
 				// Record first VCS we find.
 				// If allowNesting is false (as it is in GOPATH), keep looking for
 				// repositories in parent directories and report an error if one is

src/cmd/go/testdata/script/version_buildvcs_git.txt

@@ -14,6 +14,14 @@ go version -m $GOBIN/a$GOEXE
 ! stdout vcs.revision
 rm $GOBIN/a$GOEXE
 
+# If there's an orphan .git file left by a git submodule, it's not a git
+# repository, and there's no VCS info.
+cd ../gitsubmodule
+go install
+go version -m $GOBIN/gitsubmodule$GOEXE
+! stdout vcs.revision
+rm $GOBIN/gitsubmodule$GOEXE
+
 # If there is a repository, but it can't be used for some reason,
 # there should be an error. It should hint about -buildvcs=false.
 # Also ensure that multiple errors are collected by "go list -e".
@@ -141,6 +149,16 @@ module example.com/b
 go 1.18
 -- repo/b/b.go --
 package b
+-- repo/gitsubmodule/.git --
+gitdir: ../.git/modules/gitsubmodule
+-- repo/gitsubmodule/go.mod --
+module example.com/gitsubmodule
+
+go 1.18
+-- repo/gitsubmodule/main.go --
+package main
+
+func main() {}
 -- outside/empty.txt --
 -- outside/c/go.mod --
 module example.com/c