tests: cover revision conversion logic in githelp tests

There was no test involving actual conversion of option values when they
contain a git revision name (to be converted as a hg one by
hgext.githelp.convert()). Adding one. This test would fail on Python 3
without https://phab.mercurial-scm.org/D7537.

Differential Revision: https://phab.mercurial-scm.org/D7540

tests/test-githelp.t

@@ -264,6 +264,10 b' githelp for reuse message using the the '
   $ hg githelp -- git commit --reuse-message deadbeef
   hg commit -M deadbeef
 
+githelp for reuse message using HEAD
+  $ hg githelp -- git commit --reuse-message HEAD~
+  hg commit -M .~1
+
 githelp for apply with no options
   $ hg githelp -- apply
   hg import --no-commit