Allow custom default merge message with .gitea/default_merge_message/<merge_style>_TEMPLATE.md (#18177)

* Allow custom default merge message with .gitea/MERGE_MESSAGE_<merge_style>_TEMPLATE.md

* Some improvements

* Follow some advices

* Fix bug

* Fix bug

* Fix lint

* Fix close comment

* Fix test

* Fix and docs

* Improve codes

* Update docs and remove unnecessary variables

* return error for GetDefaultMergeMessage

* Fix test

* improve code

* ignore unknow unit type

* return error for GetDefaultMergeMessage

* Update services/pull/merge.go

* Some improvements

* Follow some advices

* Fix bug

* Fix lint

* Improve codes

* Update docs and remove unnecessary variables

* return error for GetDefaultMergeMessage

* improve code

* Handle deleted HeadRepo in GetDefaultMergeMessage

Signed-off-by: Andrew Thornton <art27@cantab.net>

* Fix test

* Fix test

Co-authored-by: zeripath <art27@cantab.net>

modules/git/commit.go

@@ -17,6 +17,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/log"
+	"code.gitea.io/gitea/modules/util"
 )
 
 // Commit represents a git commit.
@@ -306,6 +307,35 @@ func (c *Commit) HasFile(filename string) (bool, error) {
 	return true, nil
 }
 
+// GetFileContent reads a file content as a string or returns false if this was not possible
+func (c *Commit) GetFileContent(filename string, limit int) (string, error) {
+	entry, err := c.GetTreeEntryByPath(filename)
+	if err != nil {
+		return "", err
+	}
+
+	r, err := entry.Blob().DataAsync()
+	if err != nil {
+		return "", err
+	}
+	defer r.Close()
+
+	if limit > 0 {
+		bs := make([]byte, limit)
+		n, err := util.ReadAtMost(r, bs)
+		if err != nil {
+			return "", err
+		}
+		return string(bs[:n]), nil
+	}
+
+	bytes, err := io.ReadAll(r)
+	if err != nil {
+		return "", err
+	}
+	return string(bytes), nil
+}
+
 // GetSubModules get all the sub modules of current revision git tree
 func (c *Commit) GetSubModules() (*ObjectCache, error) {
 	if c.submoduleCache != nil {