blame: remove unnecessary use of get_commit_info()

When `git blame --color-by-age`, the determine_line_heat() is called to
select how to color the output based on the commit's author date.  It
uses the get_commit_info() to parse the information into a `commit_info`
structure, however, this is actually unnecessary because the
determine_line_heat() caller also does the same.

Instead, let's change the determine_line_heat() to take a `commit_info`
structure and remove the internal call to get_commit_info() thus
cleaning up and optimizing the code path.

Enabling Git's trace2 API in order to record the execution time for
every call to determine_line_heat() function:

   + trace2_region_enter("blame", "determine_line_heat", the_repository);
     determine_line_heat(ent, &default_color);
   + trace2_region_enter("blame", "determine_line_heat", the_repository);

Then, running `git blame` for "kernel/fork.c" in linux.git and summing
all the execution time for every call (around 1.3k calls) resulted in
2.6x faster execution (best out 3):

   git built from 328c109 (The eighth batch, 2021-02-12) = 42ms
   git built from 328c109 + this change                  = 16ms

Signed-off-by: Rafael Silva <[email protected]>
Reviewed-by: Taylor Blau <[email protected]>
Signed-off-by: Junio C Hamano <[email protected]>

Author: raffs

builtin/blame.c

@@ -425,13 +425,11 @@ static void setup_default_color_by_age(void)
 	parse_color_fields("blue,12 month ago,white,1 month ago,red");
 }
 
-static void determine_line_heat(struct blame_entry *ent, const char **dest_color)
+static void determine_line_heat(struct commit_info *ci, const char **dest_color)
 {
 	int i = 0;
-	struct commit_info ci;
-	get_commit_info(ent->suspect->commit, &ci, 1);
 
-	while (i < colorfield_nr && ci.author_time > colorfield[i].hop)
+	while (i < colorfield_nr && ci->author_time > colorfield[i].hop)
 		i++;
 
 	*dest_color = colorfield[i].col;
@@ -453,7 +451,7 @@ static void emit_other(struct blame_scoreboard *sb, struct blame_entry *ent, int
 	cp = blame_nth_line(sb, ent->lno);
 
 	if (opt & OUTPUT_SHOW_AGE_WITH_COLOR) {
-		determine_line_heat(ent, &default_color);
+		determine_line_heat(&ci, &default_color);
 		color = default_color;
 		reset = GIT_COLOR_RESET;
 	}