tests: allow more flexibility in git diffstat output

The output of 'git diff --stat' changed in git 1.7.10 and 1.7.11.
To ensure the tests work with earlier versions of git as well,
the output is now wrapped with a whitespace regex.

tests/test-subrepo-git.t

@@ -772,9 +772,12 b' add new changes, including whitespace'
    foo
    bar (no-eol)
 
+execute a diffstat
+the output contains a regex, because git 1.7.10 and 1.7.11
+ change the amount of whitespace
   $ hg diff --subrepos --stat
-  barfoo | 1 +
-   foobar | 2 +-
+  \s*barfoo |\s*1 + (re)
+  \s*foobar |\s*2 +- (re)
    2 files changed, 2 insertions(+), 1 deletion(-) (no-eol)
 
 ensure adding include/exclude ignores the subrepo