feat: fill MR desc with all commit bodies

[guyfedwards]

Description
adds --fill-commit-body flag to add all the commit bodies to the MR description
when creating an my with --fill

closes #795

Related Issue
Resolves #795

How Has This Been Tested?
Wrote tests for this and tested on personal gitlab account

Screenshots (if appropriate):
@profclems @zemzale not sure how I feel about the formatting, ends up looking kinda weird because of the indentation

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[codecov]

Codecov Report

Merging #819 (52126c4) into trunk (b8a02b5) will decrease coverage by 0.04%.
The diff coverage is 66.66%.

@@            Coverage Diff             @@
##            trunk     #819      +/-   ##
==========================================
- Coverage   59.45%   59.41%   -0.05%     
==========================================
  Files          93       93              
  Lines        7357     7364       +7     
==========================================
+ Hits         4374     4375       +1     
- Misses       2601     2607       +6     
  Partials      382      382              

Impacted Files	Coverage Δ
commands/mr/create/mr_create.go	33.50% <66.66%> (+3.02%)	⬆️
internal/config/config_migration.go	0.00% <0.00%> (ø)
internal/config/local_config.go	31.81% <0.00%> (+5.73%)	⬆️
internal/config/file.go	100.00% <0.00%> (+12.90%)	⬆️

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[zemzale]

Nice work @guyfedwards. LGTM.

The formatting maybe is not the best, but I can't think of anything better. I am for adding a feature and improving upon it later on

[profclems]

Screenshots (if appropriate):
@profclems @zemzale not sure how I feel about the formatting, ends up looking kinda weird because of the indentation

I'm not convinced about this too. Looks really weird. To fix this in markdown, you need two spaces after the first line like so:

- first line <space><space>
this is the commit description

This will be rendered as expected:

• this is a test commit title
  this is the commit description
  and a really long one.
• this is a second test commit title
  this is the commit descriptions
  for the second commit

[profclems]

Please add usage examples here:

glab/commands/mr/create/mr_create.go

Lines 83 to 88 in 999d1ff

	Example: heredoc.Doc(`
	$ glab mr new
	$ glab mr create -a username -t "fix annoying bug"
	$ glab mr create -f --draft --label RFC
	$ glab mr create --fill --yes --web
	`),

[guyfedwards]

@profclems I have made the requested changes but github seems to be stuck thinking I haven't?