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_test.go

@@ -106,8 +106,9 @@ func TestGitDescribeTags(t *testing.T) {
 
 func TestGitRemoteURL(t *testing.T) {
 	type remote struct {
-		name string
-		url  string
+		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()