normalize_path_copy(): fix pushing to //server/share/dir on Windows

normalize_path_copy() is not prepared to keep the double-slash of a
//server/share/dir kind of path, but treats it like a regular POSIX
style path and transforms it to /server/share/dir.

The bug manifests when 'git push //server/share/dir master' is run,
because tmp_objdir_add_as_alternate() uses the path in normalized
form when it registers the quarantine object database via
link_alt_odb_entries(). Needless to say that the directory cannot be
accessed using the wrongly normalized path.

Fix it by skipping all of the root part, not just a potential drive
prefix. offset_1st_component takes care of this, see the
implementation in compat/mingw.c::mingw_offset_1st_component().

Signed-off-by: Johannes Sixt <j6t@kdbg.org>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

path.c

@@ -961,7 +961,7 @@ const char *remove_leading_path(const char *in, const char *prefix)
  *
  * Performs the following normalizations on src, storing the result in dst:
  * - Ensures that components are separated by '/' (Windows only)
- * - Squashes sequences of '/'.
+ * - Squashes sequences of '/' except "//server/share" on Windows
  * - Removes "." components.
  * - Removes ".." components, and the components the precede them.
  * Returns failure (non-zero) if a ".." component appears as first path
@@ -984,17 +984,22 @@ const char *remove_leading_path(const char *in, const char *prefix)
 int normalize_path_copy_len(char *dst, const char *src, int *prefix_len)
 {
 	char *dst0;
-	int i;
+	const char *end;
 
-	for (i = has_dos_drive_prefix(src); i > 0; i--)
-		*dst++ = *src++;
+	/*
+	 * Copy initial part of absolute path: "/", "C:/", "//server/share/".
+	 */
+	end = src + offset_1st_component(src);
+	while (src < end) {
+		char c = *src++;
+		if (is_dir_sep(c))
+			c = '/';
+		*dst++ = c;
+	}
 	dst0 = dst;
 
-	if (is_dir_sep(*src)) {
-		*dst++ = '/';
-		while (is_dir_sep(*src))
-			src++;
-	}
+	while (is_dir_sep(*src))
+		src++;
 
 	for (;;) {
 		char c = *src;