Remove the awkward default value marker from --symlink_prefix.

The wording of the help text is somewhat awkward, but we cannot mention "Bazel" explicitly because the string "Bazel" comes from Constants.java .

--
MOS_MIGRATED_REVID=101664076

src/main/java/com/google/devtools/build/lib/buildtool/BuildRequest.java

diff --git a/src/main/java/com/google/devtools/build/lib/buildtool/BuildRequest.java b/src/main/java/com/google/devtools/build/lib/buildtool/BuildRequest.java
index a506c44..718b7f8 100644
--- a/src/main/java/com/google/devtools/build/lib/buildtool/BuildRequest.java
+++ b/src/main/java/com/google/devtools/build/lib/buildtool/BuildRequest.java
@@ -56,8 +56,6 @@
  * as --keep_going, --jobs, etc.
  */
 public class BuildRequest implements OptionsClassProvider {
-  private static final String DEFAULT_SYMLINK_PREFIX_MARKER = "...---:::@@@DEFAULT@@@:::--...";
-
   /**
    * A converter for symlink prefixes that defaults to {@code Constants.PRODUCT_NAME} and a
    * minus sign if the option is not given.
@@ -67,9 +65,7 @@
   public static class SymlinkPrefixConverter implements Converter<String> {
     @Override
     public String convert(String input) throws OptionsParsingException {
-      return input.equals(DEFAULT_SYMLINK_PREFIX_MARKER)
-          ? Constants.PRODUCT_NAME + "-"
-          : input;
+      return input.isEmpty() ? Constants.PRODUCT_NAME + "-" : input;
     }
 
     @Override
@@ -237,12 +233,12 @@
     public boolean announce;
 
     @Option(name = "symlink_prefix",
-        defaultValue = DEFAULT_SYMLINK_PREFIX_MARKER,
+        defaultValue = "",
         converter = SymlinkPrefixConverter.class,
         category = "misc",
         help = "The prefix that is prepended to any of the convenience symlinks that are created "
             + "after a build. If '/' is passed, then no symlinks are created and no warning is "
-            + "emitted."
+            + "emitted. If omitted or is empty, the default value is the name of the build tool."
         )
     public String symlinkPrefix;