fix: git worktree was not supported

[i4ki]

Reason for This Change

We were using git rev-parse --git-dir to found the .git directory and then infer the project root but this approach doesn't work with git worktree.

We are not officially announcing support for it yet as we need to better test if all Terramate features work well with worktree, but this change in one step in that direction.

Description of Changes

It was simply changed to use git rev-parse --show-toplevel.

[codecov]

Codecov Report

Base: 71.68% // Head: 71.72% // Increases project coverage by +0.03% 🎉

Coverage data is based on head (8f35af8) compared to base (13c43a8).
Patch coverage: 50.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #813      +/-   ##
==========================================
+ Coverage   71.68%   71.72%   +0.03%     
==========================================
  Files          81       81              
  Lines       13043    13039       -4     
==========================================
+ Hits         9350     9352       +2     
+ Misses       3333     3327       -6     
  Partials      360      360              

Flag	Coverage Δ
tests	71.72% <50.00%> (+0.03%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/terramate/cli/cli.go	0.00% <0.00%> (ø)
git/git.go	75.77% <100.00%> (+0.34%)	⬆️

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[mariux]

lgtm