monoblue: make the size of line links bigger to cover line numbers better

Due to how the line numbers in monoblue are formed (via css counters), the size
of the area with the numbers and the size of the actually clickable links are
not tied together well enough. Before this patch, there were noticeable "gaps"
between line links - clicking on the bottom part of a visible line number did
nothing as opposed to selecting this line.

Let's set font-size for everything in pre.sourcelines so that it also affects
the links and then add a bit of padding to them so compensate for layout
differences.

This way the sizes are still not 100% the same, but should be very close.

mercurial/templates/static/style-monoblue.css

@@ -262,13 +262,16 b' span.difflineplus { color:#008800; }'
 span.difflineminus { color:#cc0000; }
 span.difflineat { color:#990099; }
 
-pre.sourcelines { position: relative; counter-reset: lineno; }
+pre.sourcelines {
+    position: relative;
+    counter-reset: lineno;
+    font-size: 1.2em;
+}
 pre.sourcelines > span {
     display: inline-block;
     box-sizing: border-box;
     width: 100%;
     padding: 0 0 0 5em;
-    font-size: 1.2em;
     counter-increment: lineno;
     vertical-align: top;
 }
@@ -295,6 +298,7 b' pre.sourcelines > a {'
     left: 0px;
     width: 4em;
     height: 1em;
+    padding: 0.15em;
 }
 pre.sourcelines.stripes > :nth-child(4n+1) { background-color: #F1F6F7; }
 pre.sourcelines.stripes > :nth-child(4n+3) { background-color: #FFFFFF; }