hgweb: fix a stupid KeyError introduced in a0e20a5eba3c

Thanks to Christian Ebert for finding this. Also added a test to prevent this
from happening again.

mercurial/hgweb/hgweb_mod.py

@@ -212,7 +212,7 b' class hgweb(object):'
                     req.form['cmd'] = [tmpl.cache['default']]
                     cmd = req.form['cmd'][0]
 
-                if cmd == 'file' and 'raw' in req.form['style']:
+                if cmd == 'file' and 'raw' in req.form.get('style', []):
                     webcommands.rawfile(self, req, tmpl)
                 else:
                     getattr(webcommands, cmd)(self, req, tmpl)

tests/test-hgweb-commands

@@ -27,6 +27,7 b' echo % File-related'
 "$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '/file/1/foo/?style=raw'
 "$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '/annotate/1/foo/?style=raw'
 "$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '/file/1/?style=raw'
+"$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '/file/1/foo' | sed "s/[0-9]* years/many years/"
 "$TESTDIR/get-with-headers.py" 127.0.0.1:$HGPORT '/filediff/1/foo/?style=raw'
 
 echo % Overviews