cygwin: allow pushing to UNC paths

cygwin can use an UNC path like //server/share/repo

 $ cd //server/share/dir
 $ mkdir test
 $ cd test
 $ git init --bare

 However, when we try to push from a local Git repository to this repo,
 there is a problem: Git converts the leading "//" into a single "/".

 As cygwin handles an UNC path so well, Git can support them better:

 - Introduce cygwin_offset_1st_component() which keeps the leading "//",
   similar to what Git for Windows does.

 - Move CYGWIN out of the POSIX in the tests for path normalization in t0060

Signed-off-by: Torsten Bögershausen <tboegi@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

compat/cygwin.c

@@ -0,0 +1,19 @@
+#include "../git-compat-util.h"
+#include "../cache.h"
+
+int cygwin_offset_1st_component(const char *path)
+{
+	const char *pos = path;
+	/* unc paths */
+	if (is_dir_sep(pos[0]) && is_dir_sep(pos[1])) {
+		/* skip server name */
+		pos = strchr(pos + 2, '/');
+		if (!pos)
+			return 0; /* Error: malformed unc path */
+
+		do {
+			pos++;
+		} while (*pos && pos[0] != '/');
+	}
+	return pos + is_dir_sep(*pos) - path;
+}

compat/cygwin.h

@@ -0,0 +1,2 @@
+int cygwin_offset_1st_component(const char *path);
+#define offset_1st_component cygwin_offset_1st_component

config.mak.uname

@@ -181,6 +181,7 @@ ifeq ($(uname_O),Cygwin)
 	UNRELIABLE_FSTAT = UnfortunatelyYes
 	SPARSE_FLAGS = -isystem /usr/include/w32api -Wno-one-bit-signed-bitfield
 	OBJECT_CREATION_USES_RENAMES = UnfortunatelyNeedsTo
+	COMPAT_OBJS += compat/cygwin.o
 endif
 ifeq ($(uname_S),FreeBSD)
 	NEEDS_LIBICONV = YesPlease

git-compat-util.h

@@ -189,6 +189,9 @@
 #include <sys/sysctl.h>
 #endif
 
+#if defined(__CYGWIN__)
+#include "compat/cygwin.h"
+#endif
 #if defined(__MINGW32__)
 /* pull in Windows compatibility stuff */
 #include "compat/mingw.h"

t/t0060-path-utils.sh

@@ -70,6 +70,8 @@ ancestor() {
 case $(uname -s) in
 *MINGW*)
 	;;
+*CYGWIN*)
+	;;
 *)
 	test_set_prereq POSIX
 	;;