diff --no-index: fix logic for paths ending in '/'

diff --no-index: fix logic for paths ending in '/'

If one of the two provided paths for git diff --no-index ends in a '/',
a failure similar to the following occurs:

  $ git diff --no-index -- /tmp/ /tmp/ ':!'
  fatal: `pos + len' is too far after the end of the buffer

This occurs because of an incorrect calculation of the skip lengths in
diff_no_index(). The code wants to calculate the length of the string,
but add one in case the string doesn't end with a slash.

The method it uses is incorrect, as it always checks the trailing NUL
character of the string. This will never be a '/', so we always add one.
In the event that we *do* have a trailing slash, this will create an
off-by-one length error later when using the skip value.

The most straightforward fix would be to correct the skip1 and skip2
lengths by using ends_with().

However, Johannes made a good point that the existing logic is wasting a
lot of computation. We generate the match string by copying the path in
and then skipping almost all of it immediately with a potentially
expensive memmove() from the strbuf_remove() call. We also re-initialize
the match stringbuf each time we call read_directory_contents.

The read_directory_contents really wants a path that is rooted at the
start of the directory scan. We're currently building this by taking the
full path and stripping out the start portion. Instead, replace this
logic by building up the portion of the match as we go.

Start by initializing two strbuf in diff_no_index containing the empty
string. Pass these into queue_diff, which in turn passes the appropriate
left or right side into read_directory_contents.

As before, we build up the matches by appending elements to the match
path and then clearing them using strbuf_setlen.

In the recursive portion of the queue_diff algorithm, we build up new
match paths the same way that we build up new buffer paths, by appending
the elements and then clearing them with strbuf_setlen after each
iteration. This is cheaper as it avoids repeated allocations, and is a
bit simpler to track what is going on.

Add a couple of test cases that pass in paths already ending in '/', to
ensure the tests cover this regression.

Reported-by: Johannes Schindelin <Johannes.Schindelin@gmx.de>
Closes: https://lore.kernel.org/git/c75ec5f9-407a-6555-d4fb-bb629d54ec61@gmx.de/
Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
[jc: small leakfixes at the end of diff_no_index()]
Signed-off-by: Junio C Hamano <gitster@pobox.com>