Add script for diffing two fish profile runs

Came in handy for tracking down the performance regression in #5219. This will
take the output of two (necessarily identical) `fish --profile ...` runs and
produce a third profile log in which all times are the difference between the
first and the second profile provided.

(I'm not sure if build_tools is the right place for it, but I think it's OK?)

build_tools/diff_profiles.fish

@@ -0,0 +1,46 @@
+#!/usr/bin/env fish
+#
+# Compares the output of two fish profile runs and emits the time difference between
+# the first and second set of results.
+#
+# Usage: ./diff_profiles.fish profile1.log profile2.log > profile_diff.log
+
+set profile1 (cat $argv[1])
+set profile2 (cat $argv[2])
+
+set line_no 0
+while set next_line_no (math $line_no + 1) && set -q profile1[$next_line_no] && set -q profile2[$next_line_no]
+	set line_no $next_line_no
+
+	set line1 $profile1[$line_no]
+	set line2 $profile2[$line_no]
+
+	if not string match -qr '^\d+\t\d+' $line1
+		echo $line1
+		continue
+	end
+
+	set results1 (string match -r '^(\d+)\t(\d+)\s+(.*)' $line1)
+	set results2 (string match -r '^(\d+)\t(\d+)\s+(.*)' $line2)
+
+	# times from both files
+	set time1 $results1[2..3]
+	set time2 $results2[2..3]
+
+	# leftover from both files
+	set remainder1 $results1[4]
+	set remainder2 $results2[4]
+
+	if not string match -q -- $remainder1 $remainder2
+		echo Mismatch on line $line_no:
+		echo - $remainder1
+		echo + $remainder2
+		exit 1
+	end
+
+	set -l diff
+	set diff[1] (math $time1[1] - $time2[1])
+	set diff[2] (math $time1[2] - $time2[2])
+
+	echo $diff[1] $diff[2] $remainder1
+end