Skip t1300.70 and 71 on msysGit.

Skip t1300.70 and 71 on msysGit.

These two tests fail on msysGit because /dev/null is an alias for nul on
Windows and when reading the value back from git config the alias does
not match the real filename. Also the HOME environment variable has a
unix-style path but git returns a native equivalent path for '~'.  As
these are platform-dependent equivalent results it seems simplest to
skip the test entirely.

Moves the NOT_MINGW prereq from t5503 into the test library.

Signed-off-by: Pat Thoyts <patthoyts@users.sourceforge.net>