tests: avoid git diffstat singular/plural output difference

'git diff --stat' output changed with regard to the amount
of changes/insertions/deletions shown.
In older git versions (1.7.7.6), output was shown as:
2 files changed, 2 insertions(+), 1 deletions(-)
In newer versions, output is shown as:
2 files changed, 2 insertions(+), 1 deletion(-)

This patch uses a regex to handle both cases.

tests/test-subrepo-git.t

@@ -778,7 +778,7 b' the output contains a regex, because git'
   $ hg diff --subrepos --stat
   \s*barfoo |\s*1 + (re)
   \s*foobar |\s*2 +- (re)
-   2 files changed, 2 insertions(+), 1 deletion(-) (no-eol)
+   2 files changed, 2 insertions\(\+\), 1 deletions?\(-\) \(no-eol\) (re)
 
 ensure adding include/exclude ignores the subrepo
   $ hg diff --subrepos -I s/foobar