Fetch using short commit SHA (#160)

## Description
This relates to woodpecker-ci/woodpecker#3965
and closes woodpecker-ci/woodpecker#3932.

The Bitbucket Pull Request Hook returns a short commit SHA. The git
fetch command requires a full-length SHA. Add a clause to detect when
the SHA1 has less than 40 characters. Fetch & switch the branch to reset
the expected commit.

Raise an error if the branch value is not defined.

## Test

Run script:

```shell
export GOOS=linux
export GOARCH=amd64
export CGO_ENABLED=0
export GO111MODULE=on
export CI_REPO_CLONE_URL=https://{User}:{Token}@bitbucket.org/{Workspace}/{Project}.git
export CI_WORKSPACE=./woodpecker
export CI_BUILD_EVENT=push
export CI_COMMIT_SHA=692972aabfec
export CI_COMMIT_BRANCH=test

./release/linux/amd64/plugin-git
```
Output:

```shell
+ git init -b test
Initialised empty Git repository in /home/pc415/bit2me/plugin-git/woodpecker/.git/
+ git config --global --replace-all safe.directory ./woodpecker
+ git remote add origin https://{User}:{Token}@bitbucket.org/{Workspace}/{Project}.git
+ git fetch --no-tags --depth=1 --filter=tree:0 origin +test:
warning: filtering not recognized by server, ignoring
From https://bitbucket.org/{Workspace}/{Project}.git
 * branch            test       -> FETCH_HEAD
 * [new branch]      test       -> origin/test
+ git switch -q test
+ git reset --hard -q 692972aabfec
+ git submodule update --init --recursive --depth=1 --recommend-shallow
+ git lfs fetch
```

plugin.go

@@ -81,6 +81,15 @@ func (p Plugin) Exec() error {
 		fmt.Println("using head checkout")
 		cmds = append(cmds, fetch(p.Pipeline.Ref, p.Config.Tags, p.Config.Depth, p.Config.filter))
 		cmds = append(cmds, checkoutHead())
+	} else if len(p.Pipeline.Commit) != 40 && len(p.Pipeline.Commit) != 64 {
+		// fetch requires full SHA1 (40 chars) or SHA256 (64 chars) commits (unambiguous reference)
+		// for short SHA1 or SHA256, fetch and switch the branch before commit reset
+		if p.Config.Branch == "" {
+			return fmt.Errorf("short commit SHA1 checkout requires a branch")
+		}
+		cmds = append(cmds, fetch(p.Config.Branch, p.Config.Tags, p.Config.Depth, p.Config.filter))
+		cmds = append(cmds, switchBranch(p.Config.Branch))
+		cmds = append(cmds, checkoutSha(p.Pipeline.Commit))
 	} else {
 		// fetch and checkout by commit sha
 		cmds = append(cmds, fetch(p.Pipeline.Commit, p.Config.Tags, p.Config.Depth, p.Config.filter))
@@ -256,6 +265,16 @@ func checkoutSha(commit string) *exec.Cmd {
 	), defaultEnvVars...)
 }
 
+// Switch executes a git switch command.
+func switchBranch(branch string) *exec.Cmd {
+	return appendEnv(exec.Command(
+		"git",
+		"switch",
+		"-q",
+		branch,
+	), defaultEnvVars...)
+}
+
 func fetchLFS() *exec.Cmd {
 	return appendEnv(exec.Command(
 		"git", "lfs",

plugin_test.go

@@ -98,6 +98,17 @@ var commits = []struct {
 		file: "README",
 		data: "Hello World!\n\nsomething is changed!\n",
 	},
+	// checkout with short SHA!
+	{
+		path:   "octocat/Hello-World",
+		clone:  "https://github.com/octocat/Hello-World.git",
+		event:  "pull_request",
+		branch: "test",
+		commit: "7629413",
+		ref:    "",
+		file:   "README",
+		data:   "Hello World!\n",
+	},
 	// ### test lfs, please do not change order, otherwise TestCloneNonEmpty will fail ###
 	// checkout with lfs skip
 	{
@@ -143,6 +154,7 @@ func TestClone(t *testing.T) {
 				Recursive: c.recursive,
 				Lfs:       c.lfs,
 				Home:      "/tmp",
+				Branch:    c.branch,
 			},
 		}
 
@@ -190,6 +202,7 @@ func TestCloneNonEmpty(t *testing.T) {
 				Recursive: c.recursive,
 				Lfs:       c.lfs,
 				Home:      "/tmp",
+				Branch:    c.branch,
 			},
 		}
 
@@ -372,6 +385,33 @@ func TestCustomCertFile(t *testing.T) {
 	}
 }
 
+func TestSwitchBranch(t *testing.T) {
+	testdata := []struct {
+		exp []string
+	}{
+		{
+			[]string{
+				"git",
+				"switch",
+				"-q",
+				"test",
+			},
+		},
+	}
+
+	for _, td := range testdata {
+		c := switchBranch("test")
+		if len(c.Args) != len(td.exp) {
+			t.Errorf("Expected: %s, got %s", td.exp, c.Args)
+		}
+		for i := range c.Args {
+			if c.Args[i] != td.exp[i] {
+				t.Errorf("Expected: %s, got %s", td.exp, c.Args)
+			}
+		}
+	}
+}
+
 // TestUpdateSubmodules tests if the arguments to `git submodule update`
 // are constructed properly.
 func TestUpdateSubmodulesRemote(t *testing.T) {