Update Rust crate git2 to 0.20

[renovate]

This PR contains the following updates:

Package	Type	Update	Change
git2	dependencies	minor	0.19 -> 0.20

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Release Notes

rust-lang/git2-rs (git2)

v0.20.0

Compare Source

0.19.0...0.20.0

Added

• Debug is now implemented for transport::Service
  #​1074
• Added Repository::commondir
  #​1079
• Added Repository::merge_base_octopus
  #​1088
• Restored impls for PartialOrd, Ord, and Hash for bitflags types that were inadvertently removed in a prior release.
  #​1096
• Added CheckoutBuilder::disable_pathspec_match
  #​1107
• Added PackBuilder::write
  #​1110

Changed

• ❗ Updated to libgit2 1.9.0
  #​1111
• ❗ Removed the ssh_key_from_memory Cargo feature, it was unused.
  #​1087
• ❗ Errors from Tree::walk are now correctly reported to the caller.
  #​1098
• ❗ The trace_set callback now takes a &[u8] instead of a &str.
  #​1071
• ❗ Error::last_error now returns Error instead of Option<Error>.
  #​1072

Fixed

• Fixed OdbReader::read return value.
  #​1061
• When a credential helper executes a shell command, don't pop open a console window on Windows.
  #​1075

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