mingw: support UNC in git clone file://server/share/repo

Extend the parser to accept file://server/share/repo in the way that
Windows users expect it to be parsed who are used to referring to file
shares by UNC paths of the form \\server\share\folder.

[jes: tightened check to avoid handling file://C:/some/path as a UNC
path.]

This closes git-for-windows#1264.

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>

connect.c

@@ -915,6 +915,10 @@ static enum protocol parse_connect_url(const char *url_orig, char **ret_host,
 
 	if (protocol == PROTO_LOCAL)
 		path = end;
+	else if (protocol == PROTO_FILE && *host != '/' &&
+		 !has_dos_drive_prefix(host) &&
+		 offset_1st_component(host - 2) > 1)
+		path = host - 2; /* include the leading "//" */
 	else if (protocol == PROTO_FILE && has_dos_drive_prefix(end))
 		path = end; /* "file://$(pwd)" may be "file://C:/projects/repo" */
 	else

t/t5500-fetch-pack.sh

@@ -708,13 +708,22 @@ do
 	# file with scheme
 	for p in file
 	do
-		test_expect_success "fetch-pack --diag-url $p://$h/$r" '
+		test_expect_success !MINGW "fetch-pack --diag-url $p://$h/$r" '
 			check_prot_path $p://$h/$r $p "/$r"
 		'
+		test_expect_success MINGW "fetch-pack --diag-url $p://$h/$r" '
+			check_prot_path $p://$h/$r $p "//$h/$r"
+		'
+		test_expect_success MINGW "fetch-pack --diag-url $p:///$r" '
+			check_prot_path $p:///$r $p "/$r"
+		'
 		# No "/~" -> "~" conversion for file
-		test_expect_success "fetch-pack --diag-url $p://$h/~$r" '
+		test_expect_success !MINGW "fetch-pack --diag-url $p://$h/~$r" '
 			check_prot_path $p://$h/~$r $p "/~$r"
 		'
+		test_expect_success MINGW "fetch-pack --diag-url $p://$h/~$r" '
+			check_prot_path $p://$h/~$r $p "//$h/~$r"
+		'
 	done
 	# file without scheme
 	for h in nohost nohost:12 [::1] [::1]:23 [ [:aa