mingw: work around t2300's assuming non-Windows paths

On Windows, we have to juggle two different schemes of representing
paths: the native, Windows paths (the only ones known to the main
Git executable) on the one hand, and POSIX-ish ones used by the Bash
through MSYS2's POSIX emulation layer on the other hand.

A Windows path looks like this: C:\git-sdk-64\usr\src\git. In modern
Windows, it is almost always legal to use forward slashes as directory
separators, which is the reason why the Git executable itself would use
the path C:/git-sdk-64/usr/src/git instead. The equivalent POSIX-ish
path would be: /c/git-sdk-64/usr/src/git.

This patch works around the assumption of t2300-cd-to-toplevel.sh that
`git --exec-path` spits out a POSIX-ish path, by converting the output
accordingly.

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

Author: dscho

t/t2300-cd-to-toplevel.sh

@@ -4,11 +4,19 @@ test_description='cd_to_toplevel'
 
 . ./test-lib.sh
 
+EXEC_PATH="$(git --exec-path)"
+test_have_prereq !MINGW ||
+case "$EXEC_PATH" in
+[A-Za-z]:/*)
+	EXEC_PATH="/${EXEC_PATH%%:*}${EXEC_PATH#?:}"
+	;;
+esac
+
 test_cd_to_toplevel () {
 	test_expect_success $3 "$2" '
 		(
 			cd '"'$1'"' &&
-			PATH="$(git --exec-path):$PATH" &&
+			PATH="$EXEC_PATH:$PATH" &&
 			. git-sh-setup &&
 			cd_to_toplevel &&
 			[ "$(pwd -P)" = "$TOPLEVEL" ]