Introduce the function strip_path_suffix()
The function strip_path_suffix() will try to strip a given suffix from
a given path. The suffix must start at a directory boundary (i.e. "core"
is not a path suffix of "libexec/git-core", but "git-core" is).
Arbitrary runs of directory separators ("slashes") are assumed identical.
Example:
prefix = strip_path_suffix("C:\\msysgit/\\libexec\\git-core",
"libexec///git-core");
will set prefix to "C:\\msysgit".
Signed-off-by: Johannes Schindelin <johannes.schindelin@gmx.de>