fix: wrong diff arguments for xcode FileMerge (#1979)

Signed-off-by: leo <longshuang@msn.cn>

Author: love-linger

src/Models/ExternalMerger.cs

@@ -50,7 +50,7 @@ static ExternalMerger()
             {
                 Supported = new List<ExternalMerger>() {
                     new ExternalMerger("git", "Use Git Settings", "", "", ""),
-                    new ExternalMerger("xcode", "FileMerge", "/usr/bin/opendiff", "\"$BASE\" \"$LOCAL\" \"$REMOTE\" -ancestor \"$MERGED\"", "\"$LOCAL\" \"$REMOTE\""),
+                    new ExternalMerger("xcode", "FileMerge", "/usr/bin/opendiff", "\"$LOCAL\" \"$REMOTE\" -ancestor \"$BASE\" -merge \"$MERGED\"", "\"$LOCAL\" \"$REMOTE\""),
                     new ExternalMerger("vscode", "Visual Studio Code", "/Applications/Visual Studio Code.app/Contents/Resources/app/bin/code", "-n --wait \"$MERGED\"", "-n --wait --diff \"$LOCAL\" \"$REMOTE\""),
                     new ExternalMerger("vscode_insiders", "Visual Studio Code - Insiders", "/Applications/Visual Studio Code - Insiders.app/Contents/Resources/app/bin/code", "-n --wait \"$MERGED\"", "-n --wait --diff \"$LOCAL\" \"$REMOTE\""),
                     new ExternalMerger("kdiff3", "KDiff3", "/Applications/kdiff3.app/Contents/MacOS/kdiff3", "\"$REMOTE\" -b \"$BASE\" \"$LOCAL\" -o \"$MERGED\"", "\"$LOCAL\" \"$REMOTE\""),