installer: do detect downgrades _to_ -rc<N> versions

The `-rc<N>` versions reduce to a registry value `CurrentVersion` that
looks like it is a _later_ version than the final version. Example:
v2.24.0.windows.1 would reduce to `2.24.0.1` while
v2.24.0.rc2.windows.1` would reduce to `2.24.0.2.1`, i.e. the -rc2
version would be mistaken for being an _upgrade_ relative to the final
v2.24.0.

To fix this, let's detect whether the version we are about to install,
or the version we want to upgrade, are potentially `-rc<N>` versions,
simply by counting the dots, and fall back to the (a little bit more
expensive) test via `git version` if that is the case.

Signed-off-by: Johannes Schindelin <[email protected]>

Author: dscho

installer/install.iss

@@ -869,15 +869,25 @@ begin
     ExitProcess(0);
 end;
 
+function CountDots(S:String):Integer;
+var
+    i:Integer;
+begin
+    Result:=0;
+    for i:=1 to Length(S) do
+        if (S[i]=#46) then
+            Result:=Result+1;
+end;
+
 function IsDowngrade(CurrentVersion,PreviousVersion:String):Boolean;
 var
     Path:String;
     i,j,CurrentLength,PreviousLength:Integer;
 begin
     Result:=(VersionCompare(CurrentVersion,PreviousVersion)<0);
 #ifdef GIT_VERSION
-    if Result then begin
-        // maybe the previous version was a prerelease?
+    if Result or (CountDots(CurrentVersion)>3) or (CountDots(PreviousVersion)>3) then begin
+        // maybe the previous version was a prerelease (prereleases have five numbers: v2.24.0-rc1.windows.1 reduces to '2.24.0.1.1')?
         if (RegQueryStringValue(HKEY_LOCAL_MACHINE,'Software\GitForWindows','InstallPath',Path))
                 and (Exec(ExpandConstant('{cmd}'),'/c ""'+Path+'\cmd\git.exe" version >"'+ExpandConstant('{tmp}')+'\previous.version""','',SW_HIDE,ewWaitUntilTerminated,i))
                 and (i=0) then begin