bash prompt: use bash builtins to find out current branch

__git_ps1() runs the '$(git symbolic-ref HEAD)' command substitution
to find out whether we are on a branch and to find out the name of
that branch.  This imposes the overhead of fork()ing a subshell and
fork()+exec()ing a git process.

Since HEAD is in most cases a single-line file and the symbolic ref
format is quite simple to recognize and parse, read and parse it using
only bash builtins, thereby sparing all that fork()+exec() overhead.
Don't display the git prompt if reading HEAD fails, because a readable
HEAD is required for a git repository.  HEAD can also be a symlink
symbolic ref (due to 'core.preferSymlinkRefs'), so use bash builtins
for reading HEAD only when HEAD is not a symlink.

Signed-off-by: SZEDER Gábor <szeder@ira.uka.de>

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