Document why we look for "git" and "gerrit" and add the command prefix

Newer git versions will ask for "git receive-pack" while older ones will
ask for "git-receive-pack".  Either variant will usually invoke the right
command, but at some point in the future the hypen form will stop being
used and only the "git foo" form is used.  Internally we want to use the
hypen form, as it simplifies our command table.  So we expand "git foo"
into "git-foo" and then do the lookup.

Signed-off-by: Shawn O. Pearce <sop@google.com>

appjar/src/main/java/com/google/gerrit/server/ssh/GerritCommandFactory.java

@@ -60,6 +60,9 @@ class GerritCommandFactory implements CommandFactory {
       args = "";
     }
 
+    // Support newer-style "git receive-pack" requests by converting
+    // to the older-style "git-receive-pack".
+    //
     if ("git".equals(cmd) || "gerrit".equals(cmd)) {
       cmd += "-";
       final int sp2 = args.indexOf(' ');