hgweb: add searchhint to templates/coal/map

coal style uses every template (except header.tmpl) directly from paper style,
but doesn't use paper/map file. Elements defined in such map files are used in
templates as you would expect. For example, paper/search.tmpl contains
'{searchhint}' and template engine replaces that with the actual hint. But when
coal style reuses paper/search.tmpl, it needs to define searchhint in its map
file as well, or template engine will not find it. So let's copy it from
paper/map to coal/map.

Before this change, if the coal style was selected, the hint for the search
field in page header was present, but it was completely empty. Although the
absence of searchhint in coal/map produced no error.

mercurial/templates/coal/map

@@ -237,3 +237,6 b' error = ../paper/error.tmpl'
 urlparameter = '{separator}{name}={value|urlescape}'
 hiddenformentry = '<input type="hidden" name="{name}" value="{value|escape}" />'
 breadcrumb = '&gt; <a href="{url|urlescape}">{name|escape}</a> '
+
+searchhint = 'Find changesets by keywords (author, files, the commit message), revision
+  number or hash, or <a href="{url|urlescape}help/revsets">revset expression</a>.'