tests: add test of for hash reference translation by `hg convert`

The convert extension translates commit references in the commit
message. We didn't have any explicit testing of this before, so let's
add a test.

Differential Revision: https://phab.mercurial-scm.org/D6212

tests/test-convert-hg-source.t

@@ -206,3 +206,19 b' manifest -r tip'
   a
   c
   d
+  $ cd ..
+
+  $ hg init commit-references
+  $ cd commit-references
+  $ echo a > a
+  $ hg ci -Aqm initial
+  $ echo b > b
+  $ hg ci -Aqm 'the previous commit was 1451231c8757'
+
+  $ cd ..
+  $ hg convert commit-references new-commit-references -q \
+  >     --config convert.hg.sourcename=yes
+  $ cd new-commit-references
+  $ hg log -T '{node|short} {desc}\n'
+  642508659503 the previous commit was c2491f685436
+  c2491f685436 initial