Patch id tests and diff performance optimization

Hi,

I finally came around to de-bashify my patch ID test script. Here we go.

[PATCH 1/3] add rebase patch id tests
[PATCH 2/3] do not search functions for patch ID
[PATCH 3/3] use cache for function names in hunk headers

The first patch adds correctness and (optional) performance tests for the patch
"hash binary sha1 into patch id"
http://thread.gmane.org/gmane.comp.version-control.git/153468/focus=155919 .

The test reveals a performance problem with the search for function names for
the hunk headers. This is fixed for patch ID computation by the second patch
and for diff in general by the third patch.

Clemens
---

 diff.c                     |    2 +-
 t/t3419-rebase-patch-id.sh |  109 ++++++++++++++++++++++++++++++++++++++++++++
 xdiff/xemit.c              |   44 +++++++++++++-----
 3 files changed, 142 insertions(+), 13 deletions(-)