In order to portably handle the difference in how 'sed -i' works between *BSD and Linux, provide a wrapper to invoke the underying system sed(1) based on which OSTYPE is in use.

To minimise the amount of churn between the difference in date(1) and ln(n) semantics, use gdate and gln from coreutils.

GNU ln uses -T instead of -h for no-target-directory. OK thomas.adam

This patch fixes test failures related the date(1) utility. In the long term we could add a wrapper function to detect valid options for date(1) and use the variant which works on the current OS. For now, this makes some tests pass and shows us where the problems are. OK thomas.adam

ok tracey

ok stsp@

To avoid accidents commit -F opens the prepared log message in an editor so it can be reviewed before the commit is created. For non-interactive use the -N option is required in addition to -F. ok millert@

ok stsp

ok stsp

ok tracey

Previous default output remains when test cases are run individually. ok tracey

New files were added under the wrong tree in the repository if the work tree has a path prefix. Fix this problem and catch it in the existing commit_with_path_prefix regression test.

approach suggested by ori@ and matches how git9 behaves ok semarie@ (who can now work with Rust-related Git repos containing submodules)