gitweb, monoblue: fix vertical align of spans in .sourcelines

Empty lines in file view could produce an inexplicable margin before the next
line (most noticeable in browsers on webkit/blink engine). That was making
empty lines seem taller than the rest.

Instead of using default vertical align, let's set it to 'top'.

This issue is actually present in paper, and only recently got into gitweb
(2239626369f5) and monoblue (119202d4d7a4). There's a bit more to it in paper,
so that will be dealt with in a future patch.

Recipe to see live: preferably using a webkit/blink browser, such as chromium,
browse a file with empty lines, e.g. https://selenic.com/hg/file/3.5/README#l8
Selecting a block of text that includes empty lines will reveal white "breaks"
in the selection. Highlighted line (#l8) also shows such a break below itself.

mercurial/templates/static/style-gitweb.css

@@ -96,6 +96,7 b' pre.sourcelines > span {'
 	width: 100%;
 	padding: 0 0 0 5em;
 	counter-increment: lineno;
+	vertical-align: top;
 }
 pre.sourcelines > span:before {
 	-moz-user-select: -moz-none;

mercurial/templates/static/style-monoblue.css

@@ -270,6 +270,7 b' pre.sourcelines > span {'
     padding: 0 0 0 5em;
     font-size: 1.2em;
     counter-increment: lineno;
+    vertical-align: top;
 }
 div.source > pre.sourcelines > span {
     padding: 1px 1px 1px 5em;

tests/test-hgweb.t

@@ -340,7 +340,7 b' static file'
 
   $ get-with-headers.py --twice localhost:$HGPORT 'static/style-gitweb.css' - date etag server
   200 Script output follows
-  content-length: 6242
+  content-length: 6264
   content-type: text/css
   
   body { font-family: sans-serif; font-size: 12px; border:solid #d9d8d1; border-width:1px; margin:10px; }
@@ -441,6 +441,7 b' static file'
   	width: 100%;
   	padding: 0 0 0 5em;
   	counter-increment: lineno;
+  	vertical-align: top;
   }
   pre.sourcelines > span:before {
   	-moz-user-select: -moz-none;