convert: fix the handling of empty changlist descriptions in P4

Empty changelist descriptions are valid in Perforce. If we encounter one of
them we are currently running into an IndexError. In case of empty commit
messages set the commit message to **empty changelist description**, which
follows Perforce terminology.

hgext/convert/p4.py

@@ -161,7 +161,12 b' class p4_source(common.converter_source)'
             d = self._fetch_revision(change)
             c = self._construct_commit(d, parents)
 
-            shortdesc = c.desc.splitlines(True)[0].rstrip('\r\n')
+            descarr = c.desc.splitlines(True)
+            if len(descarr) > 0:
+                shortdesc = descarr[0].rstrip('\r\n')
+            else:
+                shortdesc = '**empty changelist description**'
+
             t = '%s %s' % (c.rev, repr(shortdesc)[1:-1])
             ui.status(util.ellipsis(t, 80) + '\n')
 

tests/test-convert-p4.t

@@ -141,5 +141,23 b' convert again'
   rev=1 desc="change a" tags="" files="a"
   rev=0 desc="initial" tags="" files="a b/c"
 
+empty commit message
+  $ p4 edit a
+  //depot/test-mercurial-import/a#3 - opened for edit
+  $ echo aaaaa >> a
+  $ p4 submit -d ""
+  Submitting change 6.
+  Locking 1 files ...
+  edit //depot/test-mercurial-import/a#4
+  Change 6 submitted.
+  $ hg convert -s p4 $DEPOTPATH dst
+  scanning source...
+  reading p4 views
+  collecting p4 changelists
+  6 **empty changelist description**
+  sorting...
+  converting...
+  0 
+
 exit trap:
   stopping the p4 server