xdiff: reduce indent heuristic overhead

xdiff: reduce indent heuristic overhead

Skip searching for better indentation heuristics if we'd slide a hunk more
than its size. This is the easiest fix proposed in the analysis[1] in
response to a patch that mercurial took for xdiff to limit searching
by a constant. Using a performance test as:

     #!python
     open('a', 'w').write(" \n" * 1000000)
     open('b', 'w').write(" \n" * 1000001)

This patch reduces the execution of "git diff --no-index a b" from
0.70s to 0.31s. However limiting the sliding to the size of the diff hunk,
which was proposed as a solution (that I found easiest to implement for
now) is not optimal for cases like

     open('a', 'w').write(" \n" * 1000000)
     open('b', 'w').write(" \n" * 2000000)

as then we'd still slide 1000000 times.

In addition to limiting the sliding to size of the hunk, also limit by a
constant. Choose 100 lines as the constant as that fits more than a screen,
which really means that the diff sliding is probably not providing a lot
of benefit anyway.

[1] https://public-inbox.org/git/72ac1ac2-f567-f241-41d6-d0f83072e0b3@alum.mit.edu/

Reported-by: Jun Wu <quark@fb.com>
Analysis-by: Michael Haggerty <mhagger@alum.mit.edu>
Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>