git-log-describe: -vfast={0|1} toggle for speed

`--contains` does exactly what I need (show which tag contains the
commit), but it is very slow when getting deeper in the history.
(500 ms vs 30000ms for a subdirectory from v3.0..v3.13-rc1 with 100
commits.

For future reproduction, the command `git log v3.0..v3.13-rc1
drivers/net/wireless/rt2x00 | git-log-describe.awk` takes
12:20 minutes to complete for the slow mode (using `--contains`) where
it is finished in only 19 seconds for the fast mode (using
`--first-parent). After the cache is filled, this time gets reduced to
slightly more than a second.

1 files changed, 14 insertions, 3 deletions

diff --git a/git-log-describe.awk b/git-log-describe.awk
index 1ab3a98..a491e13 100755
--- a/git-log-describe.awk
+++ b/git-log-describe.awk
@@ -10,6 +10,10 @@
 # -vcache_dir=DIR       Use DIR to store cached git-describe results. DIR will
 #                       be created if non-existent. Implies -vcache=1.
 # -vcache_ro={0|1}      Set to 1 to prevent creating new files in cache.
+# -vfast={0|1}          If 0 (default), then --contains will be used which
+#                       relates to the first tag containing this commit.
+#                       Otherwise, --first-parent will be used which relates to
+#                       the tag on which the commit is based.
 #
 # NOTE: when cache is enabled, one must clear it when the `git describe` options
 # are changed (see variable "opts" below) or when a new tag is added before an
@@ -28,15 +32,22 @@ BEGIN {
 			cache_dir = cache_dir "/.git/describe-cache.d";
 	}
 
+	if (fast) {
+		# Faster, but it relates to base of this commit.
+		opts = "--first-parent --tags";
+		if (cache_dir) cache_dir = cache_dir "/first-parent";
+	} else {
+		# Slower, but relates to the first tag containing this commit.
+		opts = "--contains";
+		if (cache_dir) cache_dir = cache_dir "/contains";
+	}
+
 	if (cache_dir) {
 		# Skip cache directory if non-existent and read only or if the
 		# directory cannot be created.
 		if (cache_ro && !is_dir(cache_dir) || !mkdir_p(cache_dir))
 			cache_dir = "";
 	}
-
-	# Use `git-describe --contains` to show which tag contains this commit.
-	opts = "--contains";
 }
 
 { print; }