Added new ExternalMerger - Plastic SCM (#1162)

Motivation:
https://m-pixel.com/how-to-use-plastic-scms-merge-tool-with-p4v/

src/Models/ExternalMerger.cs

@@ -42,6 +42,7 @@ namespace SourceGit.Models
                     new ExternalMerger(7, "win_merge", "WinMerge", "WinMergeU.exe", "\"$MERGED\"", "-u -e -sw \"$LOCAL\" \"$REMOTE\""),
                     new ExternalMerger(8, "codium", "VSCodium", "VSCodium.exe", "-n --wait \"$MERGED\"", "-n --wait --diff \"$LOCAL\" \"$REMOTE\""),
                     new ExternalMerger(9, "p4merge", "P4Merge", "p4merge.exe", "-tw 4 \"$BASE\" \"$LOCAL\" \"$REMOTE\" \"$MERGED\"", "-tw 4 \"$LOCAL\" \"$REMOTE\""),
+                    new ExternalMerger(10, "plastic_merge", "Plastic SCM", "mergetool.exe", "-s=\"$REMOTE\" -b=\"$BASE\" -d=\"$LOCAL\" -r=\"$MERGED\" --automatic", "-s=\"$LOCAL\" -d=\"$REMOTE\""),
                 };
             }
             else if (OperatingSystem.IsMacOS())