distrobox-enter: improve cmd composition speed

[TigerGorilla2]

TLDR:
distrobox-enter is slow. I did some timing of the code using the original but timed and slightly modified but eventually came to the improvements proposed by this PR.

Long version:

I had noticed that my command to launch a terminal

distrobox-enter home -- foot

has become slower but especially so on my laptop, taking multiple seconds.

I then eliminated foot by running time distrobox-enter home -- true. It was still slow. So I tried to launch the podman-exec manually and it only took ~250ms instead of 2-3 seconds.

Now I was pretty sure it was on distrobox' side and figured out which part of the distrobox-enter script is taking so much time using the original but timed . The slowest part is in this loop

My first idea was to replace the grep -q " " by e.g.

case ...
    *" "*) ...
    *) ...
esac

to check if a line had a space. This did improve it to close to one second total runtime (-1sec)

Timings from my machine

[david@david-laptop:/tmp/dbx-wuOy]$ git co slower
Switched to branch 'slower'

[david@david-laptop:/tmp/dbx-wuOy]$ time ./distrobox-enter home -- true
before gen-cmd took 300ms
gen-cmd took 116ms

composing cmd (bottleneck) took 1813ms
prepending container cmd took 33ms
launching the cmd in container took 289ms

real	0m2.597s
user	0m1.299s
sys	0m1.511s

[david@david-laptop:/tmp/dbx-wuOy]$ time ./distrobox-enter home -- true
before gen-cmd took 278ms
gen-cmd took 215ms

composing cmd (bottleneck) took 1994ms
prepending container cmd took 41ms
launching the cmd in container took 383ms

real	0m2.965s
user	0m1.464s
sys	0m1.728s

[david@david-laptop:/tmp/dbx-wuOy]$ git co faster
Switched to branch 'faster'

[david@david-laptop:/tmp/dbx-wuOy]$ time ./distrobox-enter home -- true
before gen-cmd took 136ms
gen-cmd took 103ms

composing cmd (bottleneck) took 644ms
prepending container cmd took 24ms
launching the cmd in container took 261ms

real	0m1.213s
user	0m0.591s
sys	0m0.677s

[david@david-laptop:/tmp/dbx-wuOy]$

I still wasn't fully satisfied and wanted to remove any ifs and cases. Which resulted in this PR. This relies on the fact, that any container-manager flag can be given using --flagname=VALUE (note the =). The changes also have the side effect of making --additional-flags accepting both styles (using = or a space) of sepearting the flag name from the value.

Some more timings including this PR (branch 'even-faster')

[nix-shell:/tmp/dbx-wuOy]$ git checkout slower
Switched to branch slower

[nix-shell:/tmp/dbx-wuOy]$ hyperfine ./distrobox-enter home -- true
Benchmark 1: ./distrobox-enter home -- true
  Time (mean ± σ):      3.673 s ±  1.024 s    [User: 1.857 s, System: 2.190 s]
  Range (min … max):    2.053 s …  4.531 s    10 runs


[nix-shell:/tmp/dbx-wuOy]$ git checkout faster
Switched to branch faster

[nix-shell:/tmp/dbx-wuOy]$ hyperfine ./distrobox-enter home -- true
Benchmark 1: ./distrobox-enter home -- true
  Time (mean ± σ):      2.141 s ±  0.693 s    [User: 1.002 s, System: 1.343 s]
  Range (min … max):    1.389 s …  3.123 s    10 runs

[nix-shell:/tmp/dbx-wuOy]$ git checkout even-faster
Switched to branch even-faster
Your branch is up to date with origin/even-faster.

[nix-shell:/tmp/dbx-wuOy]$ hyperfine ./distrobox-enter home -- true
Benchmark 1: ./distrobox-enter home -- true
  Time (mean ± σ):     633.9 ms ± 173.3 ms    [User: 420.5 ms, System: 195.1 ms]
  Range (min … max):   417.9 ms … 802.6 ms    10 runs