style: vertically align comment bubble icon

The icon of the comment bubble is slightly off-center vertically (too low).
This problem can be solved by reducing the line height instead of using the
default of 20px.

kallithea/public/less/kallithea-diff.less

@@ -183,6 +183,7 BIN_FILENODE = 6
   background: @kallithea-theme-main-color;
   width: 16px;
   height: 16px;
+  line-height: 14px;
   cursor: pointer;
   padding: 0 2px 2px 0.5px;
   border: 1px solid @kallithea-theme-main-color;