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

["René Scharfe" <l.s.r@web.de>]

"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>
---
 grep.c          | 3 ++-
 t/t7810-grep.sh | 6 +++---
 2 files changed, 5 insertions(+), 4 deletions(-)

diff --git a/grep.c b/grep.c
index c7e1dc1e0e..a54e5d86a9 100644
--- a/grep.c
+++ b/grep.c
@@ -1267,6 +1267,7 @@ static void show_line(struct grep_opt *opt,
 		regmatch_t match;
 		enum grep_context ctx = GREP_CONTEXT_BODY;
 		int eflags = 0;
+		const char *start = bol;
 
 		if (want_color(opt->color)) {
 			if (sign == ':')
@@ -1285,6 +1286,7 @@ static void show_line(struct grep_opt *opt,
 			if (match.rm_so == match.rm_eo)
 				break;
 
+			cno = bol - start + match.rm_so + 1;
 			if (opt->only_matching)
 				show_line_header(opt, name, lno, cno, sign);
 			else
@@ -1294,7 +1296,6 @@ static void show_line(struct grep_opt *opt,
 			if (opt->only_matching)
 				opt->output(opt, "\n", 1);
 			bol += match.rm_eo;
-			cno += match.rm_eo;
 			rest -= match.rm_eo;
 			eflags = REG_NOTBOL;
 		}
diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index 64ac4f04ee..bd439563d6 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh
@@ -322,11 +322,11 @@ do
 		${HC}file:1:5:mmap
 		${HC}file:2:5:mmap
 		${HC}file:3:5:mmap
-		${HC}file:3:13:mmap
+		${HC}file:3:14:mmap
 		${HC}file:4:5:mmap
-		${HC}file:4:13:mmap
+		${HC}file:4:14:mmap
 		${HC}file:5:5:mmap
-		${HC}file:5:13:mmap
+		${HC}file:5:14:mmap
 		EOF
 		git grep --column -n -o -e mmap $H >actual &&
 		test_cmp expected actual
-- 
2.54.0