tests/test-merge4.t

tests: increase timeout for slow test

Test case `test-sparse-revlog.t` need some artifact (a bundle) build before it
can run. The artifact is expensive to build, but can be reused from one run to
the other. We are about to update that test to make the artifact building
automatic if `--allow-slow-tests` is passed. However, we need a bump the
timeout a bit to make sure the artifact building as time to finish.

We could maybe teach run-tests.py how to directly handle such artifacts.
However since there is only one of them for now, this seems premature.

There are also some room to speed up the bundle creation for
test-sparse-revlog.t

  $ hg init
  $ echo This is file a1 > a
  $ hg add a
  $ hg commit -m "commit #0"
  $ echo This is file b1 > b
  $ hg add b
  $ hg commit -m "commit #1"
  $ hg update 0
  0 files updated, 0 files merged, 1 files removed, 0 files unresolved
  $ echo This is file c1 > c
  $ hg add c
  $ hg commit -m "commit #2"
  created new head
  $ hg merge 1
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  (branch merge, don't forget to commit)
  $ rm b
  $ echo This is file c22 > c

Test hg behaves when committing with a missing file added by a merge

  $ hg commit -m "commit #3"
  abort: cannot commit merge with missing files
  [255]