mingw: only use Bash-ism `builtin pwd -W` when available

Traditionally, Git for Windows' SDK uses Bash as its default shell.
However, other Unix shells are available, too. Most notably, the Win32
port of BusyBox comes with `ash` whose `pwd` command already prints
Windows paths as Git for Windows wants them, while there is not even a
`builtin` command.

Therefore, let's be careful not to override `pwd` unless we know that
the `builtin` command is available.

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

git-sh-setup.sh

@@ -306,10 +306,16 @@ case $(uname -s) in
 			/usr/bin/find "$@"
 		}
 	fi
-	# git sees Windows-style pwd
-	pwd () {
-		builtin pwd -W
-	}
+	# On Windows, Git wants Windows paths. But /usr/bin/pwd spits out
+	# Unix-style paths. At least in Bash, we have a builtin pwd that
+	# understands the -W option to force "mixed" paths, i.e. with drive
+	# prefix but still with forward slashes. Let's use that, if available.
+	if type builtin >/dev/null 2>&1
+	then
+		pwd () {
+			builtin pwd -W
+		}
+	fi
 	is_absolute_path () {
 		case "$1" in
 		[/\\]* | [A-Za-z]:*)

t/test-lib.sh

@@ -1726,10 +1726,16 @@ case $uname_s in
 			/usr/bin/find "$@"
 		}
 	fi
-	# git sees Windows-style pwd
-	pwd () {
-		builtin pwd -W
-	}
+	# On Windows, Git wants Windows paths. But /usr/bin/pwd spits out
+	# Unix-style paths. At least in Bash, we have a builtin pwd that
+	# understands the -W option to force "mixed" paths, i.e. with drive
+	# prefix but still with forward slashes. Let's use that, if available.
+	if type builtin >/dev/null 2>&1
+	then
+		pwd () {
+			builtin pwd -W
+		}
+	fi
 	# no POSIX permissions
 	# backslashes in pathspec are converted to '/'
 	# exec does not inherit the PID