gitutil: find default remote by tracking branch

Using this command resolves remote based on remote
tracking branch of the curently selected branch and
should be more precise in case we can't predict if user
uses origin to mark upstream or their fork.

Signed-off-by: Tonis Tiigi <tonistiigi@gmail.com>

util/gitutil/gitutil.go

@@ -78,7 +78,13 @@ func (c *Git) GitDir() (string, error) {
 }
 
 func (c *Git) RemoteURL() (string, error) {
-	// Try to get the remote URL from the origin remote first
+	// Try default remote based on remote tracking branch
+	if remote, err := c.currentRemote(); err == nil && remote != "" {
+		if ru, err := c.clean(c.run("remote", "get-url", remote)); err == nil && ru != "" {
+			return stripCredentials(ru), nil
+		}
+	}
+	// Next try to get the remote URL from the origin remote first
 	if ru, err := c.clean(c.run("remote", "get-url", "origin")); err == nil && ru != "" {
 		return stripCredentials(ru), nil
 	}
@@ -149,6 +155,22 @@ func (c *Git) clean(out string, err error) (string, error) {
 	return out, err
 }
 
+func (c *Git) currentRemote() (string, error) {
+	symref, err := c.clean(c.run("symbolic-ref", "-q", "HEAD"))
+	if err != nil {
+		return "", err
+	}
+	if symref == "" {
+		return "", nil
+	}
+	// git for-each-ref --format='%(upstream:remotename)'
+	remote, err := c.clean(c.run("for-each-ref", "--format=%(upstream:remotename)", symref))
+	if err != nil {
+		return "", err
+	}
+	return remote, nil
+}
+
 func IsUnknownRevision(err error) bool {
 	if err == nil {
 		return false

util/gitutil/gitutil_test.go

@@ -108,6 +108,7 @@ func TestGitRemoteURL(t *testing.T) {
 	type remote struct {
 		name     string
 		url      string
+		tracking string
 	}
 
 	cases := []struct {
@@ -165,6 +166,36 @@ func TestGitRemoteURL(t *testing.T) {
 			},
 			fail: true,
 		},
+		{
+			name: "single tracking branch",
+			remotes: []remote{
+				{
+					name:     "foo",
+					url:      "https://github.com/tonistiigi/buildx.git",
+					tracking: "master",
+				},
+			},
+			expected: "https://github.com/tonistiigi/buildx.git",
+		},
+		{
+			name: "fork tracking branch",
+			remotes: []remote{
+				{
+					name: "origin",
+					url:  "git@github.com:crazy-max/buildx.git",
+				},
+				{
+					name:     "foobranch",
+					url:      "https://github.com/tonistiigi/buildx.git",
+					tracking: "master",
+				},
+				{
+					name: "crazymax",
+					url:  "git@github.com:crazy-max/buildx.git",
+				},
+			},
+			expected: "https://github.com/tonistiigi/buildx.git",
+		},
 	}
 	for _, tt := range cases {
 		tt := tt
@@ -177,6 +208,9 @@ func TestGitRemoteURL(t *testing.T) {
 			GitCommit(c, t, "initial commit")
 			for _, r := range tt.remotes {
 				GitSetRemote(c, t, r.name, r.url)
+				if r.tracking != "" {
+					GitSetMainUpstream(c, t, r.name, r.tracking)
+				}
 			}
 
 			ru, err := c.RemoteURL()

util/gitutil/testutil.go

@@ -52,6 +52,15 @@ func GitSetRemote(c *Git, tb testing.TB, name string, url string) {
 	require.NoError(tb, err)
 }
 
+func GitSetMainUpstream(c *Git, tb testing.TB, remote, target string) {
+	tb.Helper()
+	_, err := fakeGit(c, "fetch", "--depth", "1", remote, target)
+	require.NoError(tb, err)
+
+	_, err = fakeGit(c, "branch", "--set-upstream-to", remote+"/"+target, "main")
+	require.NoError(tb, err)
+}
+
 func Mktmp(tb testing.TB) string {
 	tb.Helper()
 	folder := tb.TempDir()