test-convert-git: use a relative gitmodule url

The absolute URL was causing this error with 1.9.5 on Windows, which had a
cascading effect:

  @@ -466,22 +466,24 @@
     >   url = $TESTTMP/git-repo5
     > EOF
     $ git commit -a -m "weird white space submodule"
  -  [master *] weird white space submodule (glob)
  -   Author: nottest <[e-mail]>
  -   1 file changed, 3 insertions(+)
  +  fatal: bad config file line 6 in $TESTTMP/git-repo6/.gitmodules
  +  [128]
     $ cd ..
     $ hg convert git-repo6 hg-repo6
     initializing destination hg-repo6 repository
     scanning source...

For reasons unknown, there is still this delta on Windows:

  @@ -490,7 +490,6 @@
     $ git commit -q -m "missing .gitmodules"
     $ cd ..
     $ hg convert git-repo6 hg-repo6 --traceback
  -  fatal: Path '.gitmodules' does not exist in '*' (glob)
     initializing destination hg-repo6 repository
     scanning source...
     sorting...

tests/test-convert-git.t

@@ -463,7 +463,7 b' test non-tab whitespace .gitmodules'
   $ cat >> .gitmodules <<EOF
   > [submodule "git-repo5"]
   >   path = git-repo5
-  >   url = $TESTTMP/git-repo5
+  >   url = git-repo5
   > EOF
   $ git commit -q -a -m "weird white space submodule"
   $ cd ..