[PATCHv2] grep: use slash for path delimiter, not colon

[Phil Hord <hordp@cisco.com>]

When a commit is grepped and matching filenames are printed, grep-objects
creates the filename by prefixing the original cmdline argument to the
matched path separated by a colon.  Normally this forms a valid blob
reference to the filename, like this:

  git grep -l foo HEAD
  HEAD:some/path/to/foo.txt
      ^

But a tree path may be given to grep instead; in this case the colon is
not a valid delimiter to use since it is placed inside a path.

  git grep -l foo HEAD:some
  HEAD:some:path/to/foo.txt
           ^

The slash path delimiter should be used instead.  Fix git grep to
discern the correct delimiter so it can report valid object names.

  git grep -l foo HEAD:some
  HEAD:some/path/to/foo.txt
           ^

Also, prevent the delimiter being added twice, as happens now in these
examples:

  git grep -l foo HEAD:
  HEAD::some/path/to/foo.txt
       ^
  git grep -l foo HEAD:some/
  HEAD:some/:path/to/foo.txt
            ^

Add a test to confirm correct path forming.
---
This version is a bit more deterministic and also adds a test.

It accepts the expense of examining the path argument again to 
determine if it is a tree-ish + path rather than just a tree (commit).
The get_sha1 call occurs one extra time for each tree-ish argument,
so it's not expensive. We avoid mucking with the object_array API this
way, and also do not rely on the object-type to tell us anything about
the way the object name was spelled.

This one also adds a check to avoid duplicating an extant delimiter.

 builtin/grep.c  |  9 ++++++++-
 t/t7810-grep.sh | 15 +++++++++++++++
 2 files changed, 23 insertions(+), 1 deletion(-)

diff --git a/builtin/grep.c b/builtin/grep.c
index 03bc442..6fc418f 100644
--- a/builtin/grep.c
+++ b/builtin/grep.c
@@ -480,8 +480,15 @@ static int grep_object(struct grep_opt *opt, const struct pathspec *pathspec,
 		len = name ? strlen(name) : 0;
 		strbuf_init(&base, PATH_MAX + len + 1);
 		if (len) {
+			struct object_context ctx;
+			unsigned char sha1[20];
+			char delimiter = ':';
+			if (!get_sha1_with_context(name, 0, sha1, &ctx) &&
+				ctx.path[0]!=0)
+				delimiter='/';
 			strbuf_add(&base, name, len);
-			strbuf_addch(&base, ':');
+			if (name[len-1] != delimiter)
+				strbuf_addch(&base, delimiter);
 		}
 		init_tree_desc(&tree, data, size);
 		hit = grep_tree(opt, pathspec, &tree, &base, base.len,
diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
index f698001..2494bfc 100755
--- a/t/t7810-grep.sh
+++ b/t/t7810-grep.sh
@@ -886,6 +886,21 @@ test_expect_success 'grep -e -- -- path' '
 '
 
 cat >expected <<EOF
+HEAD:t/a/v:1:vvv
+HEAD:t/v:1:vvv
+EOF
+
+test_expect_success "grep HEAD -- path/" '
+	git grep -n -e vvv HEAD -- t/ >actual &&
+	test_cmp expected actual
+'
+
+test_expect_success "grep HEAD:path" '
+	git grep -n -e vvv HEAD:t/ >actual &&
+	test_cmp expected actual
+'
+
+cat >expected <<EOF
 hello.c:int main(int argc, const char **argv)
 hello.c:	printf("Hello world.\n");
 EOF
-- 
1.8.4.557.g34b3a2e