Add -F to commit search to treat keywords as strings (go-gitea#28744)

Fixes go-gitea#28269

The [default
behavior](https://git-scm.com/docs/git-log#Documentation/git-log.txt---basic-regexp)
of --grep in git log is to interpret the keyword as a regular
expression. This causes the search to fail in the cases where the search
keyword contains a `[`, since `[` is a special character used in grep.

If we want our keywords to be interpreted as 'strings', we should use
[-F
flag](https://git-scm.com/docs/git-log#Documentation/git-log.txt---basic-regexp).

modules/git/repo_commit.go

@@ -142,6 +142,9 @@ func (repo *Repository) searchCommits(id ObjectID, opts SearchCommitsOptions) ([
 		cmd.AddArguments("--all")
 	}
 
+	// interpret search string keywords as string instead of regex
+	cmd.AddArguments("-F")
+
 	// add remaining keywords from search string
 	// note this is done only for command created above
 	for _, v := range opts.Keywords {