setup: fix windows path buffer over-stepping

Fix a buffer over-stepping issue triggered by providing an absolute path
that is similar to the work tree path.

abspath_part_inside_repo() may currently increment the path pointer by
offset_1st_component() + wtlen, which is too much, since
offset_1st_component() is a subset of wtlen.

For the *nix-style prefix '/', this does (by luck) not cause any issues,
since offset_1st_component() is 1 and there will always be a '/' or '\0'
that can "absorb" this.

In the case of DOS-style prefixes though, the offset_1st_component() is
3 and this can potentially over-step the string buffer. For example if

    work_tree = "c:/r"
    path      = "c:/rl"

Then wtlen is 4, and incrementing the path pointer by (3 + 4) would
end up 2 bytes outside a string buffer of length 6.

Similarly if

    work_tree = "c:/r"
    path      = "c:/rl/d/a"

Then (since the loop starts by also incrementing the pointer one step),
this would mean that the function would miss checking if "c:/rl/d" could
be the work_tree, arguably this is unlikely though, since it would only
be possible with symlinks on windows.

Fix this by simply avoiding to increment by offset_1st_component() and
wtlen at the same time.

Signed-off-by: Martin Erik Werner <martinerikwerner@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

setup.c

@@ -29,7 +29,7 @@ static int abspath_part_inside_repo(char *path)
 		return -1;
 	wtlen = strlen(work_tree);
 	len = strlen(path);
-	off = 0;
+	off = offset_1st_component(path);
 
 	/* check if work tree is already the prefix */
 	if (wtlen <= len && !strncmp(path, work_tree, wtlen)) {
@@ -45,7 +45,7 @@ static int abspath_part_inside_repo(char *path)
 		off = wtlen;
 	}
 	path0 = path;
-	path += offset_1st_component(path) + off;
+	path += off;
 
 	/* check each '/'-terminated level */
 	while (*path) {