t9390: work around clash with MSYS2's Unix<->Win32 path conversion

MSYS2 tries to be very helpful, and in most cases it even works, by
converting parameters passed from inside an MSYS2 Bash to a non-MSYS2
application (such as `git.exe`) if they look like Unix-style paths or
path lists.

Sometimes, however, this automatic path conversion is unhelpful, e.g.
when passing the parameter `foo:.` to Git, which MSYS2 will readily
convert to a Windows-style path list: `foo;bar` (i.e. using a semicolon
instead of a colon).

Happily, there is a way to avoid that: the `MSYS_NO_PATHCONV` variable.
Let's use it.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

t/t9390-filter-repo.sh

@@ -1197,6 +1197,12 @@ test_expect_success 'startup sanity checks' '
 
 test_expect_success 'other startup error cases and requests for help' '
 	(
+		# prevent MSYS2 (Git for Windows) from converting the colon to
+		# a semicolon when encountering parameters that look like
+		# Unix-style, colon-separated path lists (such as `foo:.`)
+		MSYS_NO_PATHCONV=1 &&
+		export MSYS_NO_PATHCONV
+
 		git init startup_errors &&
 		cd startup_errors &&