commit: batch dump stages, do not re-read on save from the workspace

[skshetry]

Fixes #10629.

Using reproducible script from #10629 description,

#! /bin/sh

cd "$(mktemp -d)"
git init -q
dvc init -q
echo 'stages:
  stepA:
    matrix:
      x: [1,2,3,4,5,6,7]
      y: [1,2,3,4,5,6,7]
      z: [1,2,3,4,5,6,7]
    cmd: echo "substep${item.x}-${item.y}-${item.z}" > data/substep${item.x}-${item.y}-${item.z}.txt
    outs:
      - data/substep${item.x}-${item.y}-${item.z}.txt' > dvc.yaml
mkdir data
dvc repro --no-commit
time -v dvc commit -f

Before

Command being timed: "dvc commit -f"
        User time (seconds): 65.71
        System time (seconds): 0.87
        Percent of CPU this job got: 99%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 1:06.69
        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): 239632
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 0
        Minor (reclaiming a frame) page faults: 17661
        Voluntary context switches: 1049
        Involuntary context switches: 1780
        Swaps: 0
        File system inputs: 0
        File system outputs: 0
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 16384
        Exit status: 0

After

Command being timed: "dvc commit -f"
        User time (seconds): 1.14
        System time (seconds): 0.49
        Percent of CPU this job got: 98%
        Elapsed (wall clock) time (h:mm:ss or m:ss): 0:01.66
        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): 63232
        Average resident set size (kbytes): 0
        Major (requiring I/O) page faults: 0
        Minor (reclaiming a frame) page faults: 6621
        Voluntary context switches: 1046
        Involuntary context switches: 423
        Swaps: 0
        File system inputs: 0
        File system outputs: 0
        Socket messages sent: 0
        Socket messages received: 0
        Signals delivered: 0
        Page size (bytes): 16384
        Exit status: 0

TODO

• Add tests.