tests/test-diff-issue2761.t

setup: avoid attempting to invoke the system-wide hg.exe on Windows

On Windows, the executable in the current directory gets priority over anything
in $PATH (both for cmd.exe and MSYS).  That means, the former code was launching
the local hg.exe instead of the system-wide one, if it was previously built.  If
that failed, it then fell back to the local hg code, but run through python.exe.

I'm not sure what it is about ef7119cd4965, but that started throwing up a
messagebox that python37.dll couldn't be loaded.  (And indeed, python37 is not
in $PATH by default.)  Invoking the local hg via the current python avoids that.

Test issue2761

  $ hg init

  $ touch to-be-deleted
  $ hg add
  adding to-be-deleted
  $ hg ci -m first
  $ echo a > to-be-deleted
  $ hg ci -m second
  $ rm to-be-deleted
  $ hg diff -r 0

Same issue, different code path

  $ hg up -C
  1 files updated, 0 files merged, 0 files removed, 0 files unresolved
  $ touch does-not-exist-in-1
  $ hg add
  adding does-not-exist-in-1
  $ hg ci -m third
  $ rm does-not-exist-in-1
  $ hg diff -r 1