grep: fix --column --only-match for 2nd and later matches

"git grep --column --only-match" shows the 1-based column number of the
first match on each line, but confusing numbers for further matches.
Example:

   $ echo 123456789012345678901234567890 >file
   $ for d in 1 2 3 4 5 6 7 8 9 0
     do
       git grep --no-index --column --only-matching $d file |
       awk -v FS=: -v l=$d: '{l = l sprintf("%3s", $2)} END {print l}'
     done
   1:  1  2 12
   2:  2  4 14
   3:  3  6 16
   4:  4  8 18
   5:  5 10 20
   6:  6 12 22
   7:  7 14 24
   8:  8 16 26
   9:  9 18 28
   0: 10 20 30

Report the column number of each match instead:

   $ for d in 1 2 3 4 5 6 7 8 9 0
     do
       ./git grep --no-index --column --only-matching $d file |
       awk -v FS=: -v l=$d: '{l = l sprintf("%3s", $2)} END {print l}'
     done
   1:  1 11 21
   2:  2 12 22
   3:  3 13 23
   4:  4 14 24
   5:  5 15 25
   6:  6 16 26
   7:  7 17 27
   8:  8 18 28
   9:  9 19 29
   0: 10 20 30

We need to adjust the test in t7810 as well.  The file it uses has the
following five lines; I add a line highlighting the matches and a ruler
at the bottom here, to make it easier to see that the second "mmap"
indeed starts at column 14:

foo mmap bar
foo_mmap bar
foo_mmap bar mmap
foo mmap bar_mmap
foo_mmap bar mmap baz
    ====     ====
123456789 123456789 1

Reported-by: Brandon Chinn <brandonchinn178@gmail.com>
Signed-off-by: René Scharfe <l.s.r@web.de>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

0 files changed, 0 insertions, 0 deletions