rev-parse: --show-cdup

When --show-prefix is useful, sometimes it is easier to cd up to
the toplevel of the tree.  This is equivalent to:

    git rev-parse --show-prefix | sed -e 's|[^/][^/]*|..|g'

but we do not have to invoke sed for that.

Signed-off-by: Junio C Hamano <junkio@cox.net>

Documentation/git-rev-parse.txt

@@ -68,10 +68,15 @@ OPTIONS
 	Show all refs found in `$GIT_DIR/refs`.
 
 --show-prefix::
-	When the command is invoked from a directory show the
+	When the command is invoked from a subdirectory, show the
 	path of the current directory relative to the top-level
 	directory.
 
+--show-cdup::
+	When the command is invoked from a subdirectory, show the
+	path of the top-level directory relative to the current
+	directory (typically a sequence of "../", or an empty string).
+
 --since=datestring, --after=datestring::
 	Parses the date string, and outputs corresponding
 	--max-age= parameter for git-rev-list command.

rev-parse.c

@@ -216,6 +216,18 @@ int main(int argc, char **argv)
 					puts(prefix);
 				continue;
 			}
+			if (!strcmp(arg, "--show-cdup")) {
+				const char *pfx = prefix;
+				while (pfx) {
+					pfx = strchr(pfx, '/');
+					if (pfx) {
+						pfx++;
+						printf("../");
+					}
+				}
+				putchar('\n');
+				continue;
+			}
 			if (!strcmp(arg, "--git-dir")) {
 				const char *gitdir = getenv(GIT_DIR_ENVIRONMENT);
 				static char cwd[PATH_MAX];