Make truncating the debug.log file an option.

Useful for developers who need to refer to futher back in debug.log history, but who don't want to
enable the -debug option and all the verbosity that comes with that.

src/init.cpp

@@ -260,6 +260,7 @@ std::string HelpMessage()
         "  -debug                 " + _("Output extra debugging information. Implies all other -debug* options") + "\n" +
         "  -debugnet              " + _("Output extra network debugging information") + "\n" +
         "  -logtimestamps         " + _("Prepend debug output with timestamp") + "\n" +
+        "  -shrinkdebugfile       " + _("Shrink debug.log file on client startup (default: 1 when no -debug)") + "\n" +
         "  -printtoconsole        " + _("Send trace/debug info to console instead of debug.log file") + "\n" +
 #ifdef WIN32
         "  -printtodebugger       " + _("Send trace/debug info to debugger") + "\n" +
@@ -443,7 +444,7 @@ bool AppInit2()
     }
 #endif
 
-    if (!fDebug)
+    if (GetBoolArg("-shrinkdebugfile", !fDebug))
         ShrinkDebugFile();
     printf("\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n\n");
     printf("Bitcoin version %s (%s)\n", FormatFullVersion().c_str(), CLIENT_DATE.c_str());