Speed up generation of commit stats by using Rugged native methods

The previous implementation iterated across the entire patch set to determine the number of lines added, deleted, and changed. Rugged has a native method Rugged::Diff#stat that does this already, which appears to be a little faster and require less RAM than doing this ourselves.

Improves performance in #41524

Using Rugged native methods

require 'rugged'

repo = Rugged::Repository.new('.')

commit_object = repo.rev_parse('master')
original_oid = repo.rev_parse('93efff945215a4407afcaf0cba15ac601b56df0d')
diff_commits = commit_object.parents[0].diff(original_oid)

puts diff_commits.stat

Previous implementation

require 'rugged'

repo = Rugged::Repository.new('.')

commit_object = repo.rev_parse('master')
original_oid = repo.rev_parse('93efff945215a4407afcaf0cba15ac601b56df0d')
diff_commits = commit_object.parents[0].diff(original_oid)

additions = 0
deletions = 0
total = 0

diff_commits.each_patch do |p|
  additions += p.stat[0]
  deletions += p.stat[1]
  total += p.changes
end

puts [additions, deletions, total]

Performance Comparison

• Time: 1.32 s vs 1.52 s
• RAM: 144992 vs. 195104 kbytes

$ /usr/bin/time -v ruby diff_rugged_native.rb
14251
6
1070973
        Command being timed: "ruby diff_rugged_native.rb"
        User time (seconds): 1.32
        System time (seconds): 0.48
        Percent of CPU this job got: 92%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.94
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 144992
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 5
        Minor (reclaiming a frame) page faults: 17916
        Voluntary context switches: 41
        Involuntary context switches: 194
        Swaps: 0
        File system inputs: 152
        File system outputs: 0
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0

$ /usr/bin/time -v ruby diff_original.rb
6
1070973
1070979
        Command being timed: "ruby diff_original.rb"
        User time (seconds): 1.52
        System time (seconds): 0.40
        Percent of CPU this job got: 98%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.96
        Average shared text size (kbytes): 0
        Average unshared data size (kbytes): 0
        Average stack size (kbytes): 0
        Average total size (kbytes): 0
        Maximum resident set size (kbytes): 195104
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 0
        Minor (reclaiming a frame) page faults: 29048
        Voluntary context switches: 36
        Involuntary context switches: 225
        Swaps: 0
        File system inputs: 0
        File system outputs: 0
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 4096
        Exit status: 0