convert: fix the handling of empty changlist descriptions in P4

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.

--- a/hgext/convert/p4.py	Tue Mar 21 17:50:44 2017 -0700
+++ b/hgext/convert/p4.py	Wed Mar 22 14:12:58 2017 -0500
@@ -161,7 +161,12 @@
             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')
 

--- a/tests/test-convert-p4.t	Tue Mar 21 17:50:44 2017 -0700
+++ b/tests/test-convert-p4.t	Wed Mar 22 14:12:58 2017 -0500
@@ -141,5 +141,23 @@
   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