Introduce the function strip_path_suffix()

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:

	strip_path_suffix("C:\\msysgit/\\libexec\\git-core",
		"libexec///git-core", &prefix)

will set prefix to "C:\\msysgit" and return 0.

Signed-off-by: Johannes Schindelin <redacted>
Acked-by: Johannes Sixt <redacted>
Signed-off-by: Junio C Hamano <redacted>