git diff too slow for a file

[SungHyun Nam <goweol@gmail.com>]

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Hello,

If I run a attached script for bunzipped attached files, I get:
(To reduce size, I removed many lines and bzipped.)

     $ ./mk.sh
     time diff -u x3 x4 >/dev/null 2>&1

     real	0m0.011s
     user	0m0.000s
     sys	0m0.010s

     time git diff >/dev/null 2>&1

     real	0m0.193s
     user	0m0.190s
     sys	0m0.000s

     $ git version
     git version 1.7.0.2.273.gc2413

     $ diff --version
     diff (GNU diffutils) 2.8.1
     ...

Well, though the files are ascii file, they includes a random
hexa-decimal datas, so that I don't interest the diff result at
all.  But the real problem is 'rebasing took so long if the file
was changed'.  Because the git tree includes several such a file,
if they changed, rebase took some miniutes for every branch.
Such a branch includes a few lines of changes for a C source file,
though.  Now I'm waiting an hour to finish rebasing all the
branches and yet a rebasing script is running... :-(

Please help!
Thanks,
namsh

The original file has more than 180000 lines and the result is:
     $ ./mk.sh
     time diff -u x3 x4 >/dev/null 2>&1

     real	0m0.759s
     user	0m0.740s
     sys	0m0.010s

     time git diff >/dev/null 2>&1

     real	0m44.460s
     user	0m44.390s
     sys	0m0.030s


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#!/bin/bash

run_command()
{
    echo "$@"
    eval "$@"
}

run_command 'time diff -u x3 x4 >/dev/null 2>&1'

{
    rm -rf u
    mkdir u
    cd u
    cp ../x3 x
    git init
    git add .
    git ci -m x
    cp ../x4 x
} >/dev/null 2>&1

echo ''
run_command 'time git diff >/dev/null 2>&1'

exit 0