t2300: "git --exec-path" is not usable in $PATH on Windows as-is

The "git" command prepends the exec-path to the PATH environment
variable for processes it spawns.  That is how ". git-sh-setup" in
our scripted Porcelains can find the dot-sourced file in the
exec-path location that is not usually on user's PATH.

When t2300 runs, because it is not spawned by the "git" command, the
scriptlet being tested did not run with a realistic setting of PATH
environment.  It lacked the exec-path on the PATH, and failed to
find the dot-sourced file.  A recent update to t2300 attempted to
fix this, with "PATH=$(git --exec-path):$PATH", which has been the
recommended way around v1.6.0 days (a script whose original was
written before that release that survives to this day is likely to
have such a line).

However, the "git --exec-path" command outputs C:\path\to\exec\dir
(not /c/path/to/exec/dir) on Windows; the recent update failed to
consider the problem that comes from it.

Even though Git itself, when doing the equivalent internally, does
so in a platform native way (i.e. on Windows, C:\path\to\exec\dir is
prepended to the existing value of %PATH% using ';' as a component
separator), the result is further massaged by bash and gets turned
into $PATH that uses /c/path/to/exec/dir with ':' separating the
components, which is the form understood by bash, so scripted
Porcelains find commands from PATH correctly.

An end user script written in shell, however, cannot prepend
"C:\path\to\exec\dir:" to the existing value of $PATH and expect
bash to magically turn it into the form it understands.  In other
words, "PATH=$(git --exec-path):$PATH" does not work as an emulation
of what "Git" internally does to the PATH on Windows.

To correctly emulate how exec-path is prepended to the PATH
environment internally on Windows, we'd need to convert
C:\git-sdk-64\usr\src\git to at least /c\git-sdk-64\usr\src\git
ourselves before prepending it to PATH.

Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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" ]