Prevent server 500 on compare branches with no common history (#6555)

* Prevent 500 if there is no common mergebase
* Prevent creation of PR with no history

modules/git/repo_pull.go

@@ -48,10 +48,8 @@ func (repo *Repository) GetPullRequestInfo(basePath, baseBranch, headBranch stri
 
 	prInfo := new(PullRequestInfo)
 	prInfo.MergeBase, err = repo.GetMergeBase(remoteBranch, headBranch)
-	if err != nil {
-		return nil, fmt.Errorf("GetMergeBase: %v", err)
-	}
-
+	if err == nil {
+		// We have a common base
 		logs, err := NewCommand("log", prInfo.MergeBase+"..."+headBranch, prettyLogFormat).RunInDirBytes(repo.Path)
 		if err != nil {
 			return nil, err
@@ -60,6 +58,13 @@ func (repo *Repository) GetPullRequestInfo(basePath, baseBranch, headBranch stri
 		if err != nil {
 			return nil, fmt.Errorf("parsePrettyFormatLogToList: %v", err)
 		}
+	} else {
+		prInfo.Commits = list.New()
+		prInfo.MergeBase, err = GetFullCommitID(repo.Path, remoteBranch)
+		if err != nil {
+			prInfo.MergeBase = remoteBranch
+		}
+	}
 
 	// Count number of changed files.
 	stdout, err := NewCommand("diff", "--name-only", remoteBranch+"..."+headBranch).RunInDir(repo.Path)

options/locale/locale_en-US.ini

@@ -684,6 +684,7 @@ editor.cannot_commit_to_protected_branch = Cannot commit to protected branch '%s
 
 commits.desc = Browse source code change history.
 commits.commits = Commits
+commits.no_commits = No commits in common. '%s' and '%s' have entirely different histories.
 commits.search = Search commits…
 commits.find = Search
 commits.search_all = All Branches

templates/repo/commits_table.tmpl

@@ -1,9 +1,13 @@
 <h4 class="ui top attached header">
 	<div class="ui stackable grid">
-		<div class="six wide column">
+		<div class="ten wide column">
+			{{if or .PageIsCommits (gt .CommitCount 0)}}
 				{{.CommitCount}} {{.i18n.Tr "repo.commits.commits"}} {{if .Branch}}({{.Branch}}){{end}}
+			{{else}}
+				{{.i18n.Tr "repo.commits.no_commits" $.BaseBranch $.HeadBranch }} {{if .Branch}}({{.Branch}}){{end}}
+			{{end}}
 		</div>
-		<div class="ten wide right aligned column">
+		<div class="six wide right aligned column">
 			{{if .PageIsCommits}}
 				<form class="ignore-dirty" action="{{.RepoLink}}/commits/{{.BranchNameSubURL | EscapePound}}/search">
 					<div class="ui tiny search input">
@@ -23,7 +27,7 @@
 	</div>
 </h4>
 
-{{if .Commits}}
+{{if and .Commits (gt .CommitCount 0)}}
 	<div class="ui attached table segment">
 		<table class="ui very basic striped fixed table single line" id="commits-table">
 			<thead>

templates/repo/pulls/compare.tmpl

@@ -54,6 +54,9 @@
 				<div class="ui segment">
 					{{.i18n.Tr "repo.pulls.has_pull_request" $.RepoLink $.RepoRelPath .PullRequest.Index | Safe}}
 				</div>
+			{{else if eq .CommitCount 0 }}
+				{{template "repo/commits_table" .}}
+				{{template "repo/diff/box" .}}
 			{{else}}
 				{{template "repo/issue/new_form" .}}
 				{{template "repo/commits_table" .}}