xdiff: reduce indent heuristic overhead...
xdiff: reduce indent heuristic overhead
Adds some threshold to avoid expensive cases, like:
```
#!python
open('a', 'w').write(" \n" * 1000000)
open('b', 'w').write(" \n" * 1000001)
```
The indent heuristic is O(N * 20) (N = 1000000) for the above case, and
makes diff much slower.
Before this patch (system git 2.14.2):
```
git diff --no-indent-heuristic a b 0.21s user 0.03s system 100% cpu 0.239 total
git diff --indent-heuristic a b 0.77s user 0.02s system 99% cpu 0.785 total
```
After this patch (git 2fc74f41, with xdiffi.c patched):
```
# with the changed xdiffi.c
git diff --indent-heuristic a b 0.16s user 0.01s system 90% cpu 0.188 total
git diff --no-indent-heuristic a b 0.18s user 0.01s system 99% cpu 0.192 total
```
Now turning on indent-heuristic has no visible impact on performance.
Differential Revision:
https://phab.mercurial-scm.org/D2601
