describe: Add --first-parent option

Only consider the first parent commit when walking the commit history. This
is useful if you only wish to match tags on your branch after a merge.

Signed-off-by: Mike Crowe <mac@mcrowe.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

Documentation/git-describe.txt

@@ -88,6 +88,11 @@ OPTIONS
 --always::
 	Show uniquely abbreviated commit object as fallback.
 
+--first-parent::
+	Follow only the first parent commit upon seeing a merge commit.
+	This is useful when you wish to not match tags on branches merged
+	in the history of the target commit.
+
 EXAMPLES
 --------
 
@@ -149,7 +154,9 @@ is found, its name will be output and searching will stop.
 If an exact match was not found, 'git describe' will walk back
 through the commit history to locate an ancestor commit which
 has been tagged.  The ancestor's tag will be output along with an
-abbreviation of the input committish's SHA-1.
+abbreviation of the input committish's SHA-1. If '--first-parent' was
+specified then the walk will only consider the first parent of each
+commit.
 
 If multiple tags were found during the walk then the tag which
 has the fewest commits different from the input committish will be

builtin/describe.c

@@ -21,6 +21,7 @@ static int debug;	/* Display lots of verbose info */
 static int all;	/* Any valid ref can be used */
 static int tags;	/* Allow lightweight tags */
 static int longformat;
+static int first_parent;
 static int abbrev = -1; /* unspecified */
 static int max_candidates = 10;
 static struct hash_table names;
@@ -336,6 +337,9 @@ static void describe(const char *arg, int last_one)
 				commit_list_insert_by_date(p, &list);
 			p->object.flags |= c->object.flags;
 			parents = parents->next;
+
+			if (first_parent)
+				break;
 		}
 	}
 
@@ -404,6 +408,7 @@ int cmd_describe(int argc, const char **argv, const char *prefix)
 		OPT_BOOLEAN(0, "all",        &all, N_("use any ref")),
 		OPT_BOOLEAN(0, "tags",       &tags, N_("use any tag, even unannotated")),
 		OPT_BOOLEAN(0, "long",       &longformat, N_("always use long format")),
+		OPT_BOOLEAN(0, "first-parent", &first_parent, N_("only follow first parent")),
 		OPT__ABBREV(&abbrev),
 		OPT_SET_INT(0, "exact-match", &max_candidates,
 			    N_("only output exact matches"), 0),

t/t6120-describe.sh

@@ -110,6 +110,9 @@ check_describe tags/e --all HEAD^^^
 check_describe B-0-* --long HEAD^^2^
 check_describe A-3-* --long HEAD^^2
 
+check_describe c-7-* --tags
+check_describe e-3-* --first-parent --tags
+
 : >err.expect
 check_describe A --all A^0
 test_expect_success 'no warning was displayed for A' '