tests: invoke hg command indirectly from shell script to run on Windows
Before this patch, test-tag.t can't run successfully on Windows,
because:
- quoted hg command ('"hg"') prevents "hg.bat" from working correctly
(only at testing with pure Python build)
"%~f0" and "%~dp0hg" in "hg.bat" cause unexpected result in this
case. BTW, quoted "\path\to\hg" works correctly.
- "`pwd`" in the command line is expanded unexpectedly
not "C:\path\to\TESTTMP" but "C;C:\path\to\TESTTMP"