Do not clobber selection within CloneRepo command

If I carefully select a subset of the clone command (e.g., the Git
URL), leave the selection alone instead of selecting the entire
command on mouse up.

Reported-by: Yuxuan 'fishy' Wang <fishywang@google.com>
Change-Id: Iffc6532aa3c0ac27d1c320200170988d073867c1
diff --git a/gitiles-servlet/src/main/resources/com/google/gitiles/templates/RepositoryIndex.soy b/gitiles-servlet/src/main/resources/com/google/gitiles/templates/RepositoryIndex.soy
index 19461bf..b4e7062 100644
--- a/gitiles-servlet/src/main/resources/com/google/gitiles/templates/RepositoryIndex.soy
+++ b/gitiles-servlet/src/main/resources/com/google/gitiles/templates/RepositoryIndex.soy
@@ -65,7 +65,7 @@
 <div class="CloneRepo">
   <div class="CloneRepo-title">Clone this repo:</div>
   <input type="text" class="u-monospace CloneRepo-command"
-         onclick="this.focus();this.select();"
+         onclick="{literal}this.focus();if(this.selectionStart==this.selectionEnd){this.select()}{/literal}"
          readonly="readonly" value="git clone {$cloneUrl}">
 </div>