Compare branches, commits and tags with each other (#6991)

* Supports tags when comparing commits or branches

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Hide headline when only comparing and don't load unused data

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Merges compare logics to allow comparing branches, commits and tags with eachother

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Display branch or tag instead of commit when used for comparing

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Show pull request form after click on button

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Transfers relevant pull.go changes from master to compare.go

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Fixes error when comparing forks against a commit or tag

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Removes console.log from JavaScript file

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Show icon next to commit reference when comparing branch or tag

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Updates css file

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Fixes import order

* Renames template variable

* Update routers/repo/compare.go

Co-Authored-By: zeripath <art27@cantab.net>

* Update from master

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Allow short-shas in compare

* Renames prInfo to compareInfo

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Check PR permissions only if compare is pull request

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Adjusts comment

Signed-off-by: Mario Lubenka <mario.lubenka@googlemail.com>

* Use compareInfo instead of prInfo

modules/git/repo_compare.go

@@ -0,0 +1,106 @@
+// Copyright 2015 The Gogs Authors. All rights reserved.
+// Copyright 2019 The Gitea Authors. All rights reserved.
+// Use of this source code is governed by a MIT-style
+// license that can be found in the LICENSE file.
+
+package git
+
+import (
+	"bytes"
+	"container/list"
+	"fmt"
+	"io"
+	"strconv"
+	"strings"
+	"time"
+)
+
+// CompareInfo represents needed information for comparing references.
+type CompareInfo struct {
+	MergeBase string
+	Commits   *list.List
+	NumFiles  int
+}
+
+// GetMergeBase checks and returns merge base of two branches.
+func (repo *Repository) GetMergeBase(tmpRemote string, base, head string) (string, error) {
+	if tmpRemote == "" {
+		tmpRemote = "origin"
+	}
+
+	if tmpRemote != "origin" {
+		tmpBaseName := "refs/remotes/" + tmpRemote + "/tmp_" + base
+		// Fetch commit into a temporary branch in order to be able to handle commits and tags
+		_, err := NewCommand("fetch", tmpRemote, base+":"+tmpBaseName).RunInDir(repo.Path)
+		if err == nil {
+			base = tmpBaseName
+		}
+	}
+
+	stdout, err := NewCommand("merge-base", base, head).RunInDir(repo.Path)
+	return strings.TrimSpace(stdout), err
+}
+
+// GetCompareInfo generates and returns compare information between base and head branches of repositories.
+func (repo *Repository) GetCompareInfo(basePath, baseBranch, headBranch string) (_ *CompareInfo, err error) {
+	var (
+		remoteBranch string
+		tmpRemote    string
+	)
+
+	// We don't need a temporary remote for same repository.
+	if repo.Path != basePath {
+		// Add a temporary remote
+		tmpRemote = strconv.FormatInt(time.Now().UnixNano(), 10)
+		if err = repo.AddRemote(tmpRemote, basePath, true); err != nil {
+			return nil, fmt.Errorf("AddRemote: %v", err)
+		}
+		defer repo.RemoveRemote(tmpRemote)
+	}
+
+	compareInfo := new(CompareInfo)
+	compareInfo.MergeBase, err = repo.GetMergeBase(tmpRemote, baseBranch, headBranch)
+	if err == nil {
+		// We have a common base
+		logs, err := NewCommand("log", compareInfo.MergeBase+"..."+headBranch, prettyLogFormat).RunInDirBytes(repo.Path)
+		if err != nil {
+			return nil, err
+		}
+		compareInfo.Commits, err = repo.parsePrettyFormatLogToList(logs)
+		if err != nil {
+			return nil, fmt.Errorf("parsePrettyFormatLogToList: %v", err)
+		}
+	} else {
+		compareInfo.Commits = list.New()
+		compareInfo.MergeBase, err = GetFullCommitID(repo.Path, remoteBranch)
+		if err != nil {
+			compareInfo.MergeBase = remoteBranch
+		}
+	}
+
+	// Count number of changed files.
+	stdout, err := NewCommand("diff", "--name-only", remoteBranch+"..."+headBranch).RunInDir(repo.Path)
+	if err != nil {
+		return nil, err
+	}
+	compareInfo.NumFiles = len(strings.Split(stdout, "\n")) - 1
+
+	return compareInfo, nil
+}
+
+// GetPatch generates and returns patch data between given revisions.
+func (repo *Repository) GetPatch(base, head string) ([]byte, error) {
+	return NewCommand("diff", "-p", "--binary", base, head).RunInDirBytes(repo.Path)
+}
+
+// GetFormatPatch generates and returns format-patch data between given revisions.
+func (repo *Repository) GetFormatPatch(base, head string) (io.Reader, error) {
+	stdout := new(bytes.Buffer)
+	stderr := new(bytes.Buffer)
+
+	if err := NewCommand("format-patch", "--binary", "--stdout", base+"..."+head).
+		RunInDirPipeline(repo.Path, stdout, stderr); err != nil {
+		return nil, concatenateError(err, stderr.String())
+	}
+	return stdout, nil
+}