fix some backward compatibility bugs in GIT_REV

[geekosaur]

It was incorrectly displaying (in-tree) when there was no git information. Also, the spacing was slightly incompatible with older versions.

This should restore 100% compatibility with older versions of cabal-install, for scripts that parse cabal --version output.

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[geekosaur]

QA notes:

Build cabal for release (cabal --project-file=cabal.release.project build cabal).
Run the following: (assumes you have cabal-3.14.2.0 and cabal-3.16.0.0 installed via e.g. ghcup)

hilfy Z$ cabal-3.14.2.0 --version | sed 's/$/~/'
cabal-install version 3.14.2.0~
compiled using version 3.14.2.0 of the Cabal library ~
hilfy Z$ cabal-3.16.0.0 --version | sed 's/$/~/'
cabal-install version 3.16.0.0 ~
compiled using version 3.16.0.0 of the Cabal library (in-tree)~
hilfy Z$ $(cabal list-bin cabal) --version | sed 's/$/~/' 
cabal-install version 3.17.0.0~
compiled using version 3.17.0.0 of the Cabal library ~

3.14.2.0 is the original output. (I assume the extra space crept in when the Cabal library version was added.)
3.16.0.0 is the buggy output.
The final one is the fixed output; it should match 3.14.2.0 aside from the reported versions, in particular in the final spaces. Note that 3.16.0.0 has both (in-tree) and an extra space at the end of the first line.