When using git gui blame on some files in one of my repositories, and
noticing some rather depressing performance, I decided to look for some
low-hanging fruit to optimize.

I found that a lot of the time is spent in xdiff_outf continually
xreallocing and freeing memory for a string.  I decided to replace that
with a strbuf.  Unfortunately this now means that there are resources
that need to be freed after using xdiff_outf, so a new interface for
doing that needed to be created first.

The performance difference on my repository is not amazing, but quite
noticeable.  Before:

:; time git blame -M -C -C -p --incremental server.c >/dev/null
101.52user 0.17system 1:41.73elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+39561minor)pagefaults 0swaps

After:

:; time git blame -M -C -C -p --incremental server.c >/dev/null
80.38user 0.30system 1:20.81elapsed 99%CPU (0avgtext+0avgdata 0maxresident)k
0inputs+0outputs (0major+50979minor)pagefaults 0swaps

Obviously, it should improve other uses of xdiff_outf as well, though I
doubt many things hit it as hard as blame does.

 [PATCH 1/2] Make xdiff_outf_{init,release} interface
 [PATCH 2/2] Use strbuf for struct xdiff_emit_state's remainder

 builtin-blame.c   |    4 ++--
 combine-diff.c    |    4 ++--
 diff.c            |   20 ++++++++++----------
 xdiff-interface.c |   44 ++++++++++++++++++++++----------------------
 xdiff-interface.h |    6 ++++--
 5 files changed, 40 insertions(+), 38 deletions(-)

-bcd
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