bash prompt: run 'git rev-parse --git-dir' directly instead of __gitdir()

__git_ps1() finds out the path to the repository by using the __gitdir() helper function. __gitdir() is basically just a wrapper around 'git rev-parse --git-dir', extended with support for recognizing a remote repository given as argument, to use the path given on the command line, and with a few shortcuts to recognize a git repository in cwd or at $GIT_DIR quickly without actually running 'git rev-parse'. However, the former two is only necessary for the completion script but makes no sense for the bash prompt, while the latter shortcuts are performance optimizations __git_ps1() can do without (they just avoid the overhead of fork()+exec()ing a git process). Run 'git rev-parse --git-dir' directly in __git_ps1(), because it will allow this patch series to combine several $(git rev-parse ...) command substitutions in the main code path, and the overall performance benefit will far outweigh the loss of those few shortcuts in __gitdir(). Furthermore, since __gitdir() is not needed anymore for the prompt, remove it from the prompt script finally eliminating its duplication between the prompt and completion scripts. Also remove the comment from the completion script warning about this code duplication. Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>