uscan: Handle absolute USCAN_DESTDIR in git mode

Simon McVittierequested to merge
smcv/devscripts:uscan-895279 into master

An absolute path for USCAN_DESTDIR was handled everywhere else, but in git mode uscan blindly appended it to $curdir, resulting in paths like "/home/smcv/src/debian/ioquake3//home/smcv/tmp/build-area..." when used.

Signed-off-by: Simon McVittie smcv@debian.org Closes: #895279

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