treewide: reduce $PATH checks for `git' executable

treewide: reduce $PATH checks for `git' executable

Repeatedly checking $PATH for `git' when we need to call it
multiple times in quick succession doesn't seem useful.  So
avoid some expensive stat(2) syscalls to make things less bad
for systems which require expensive CPU vulnerability
mitigations.

This also saves a bunch of memory allocations since we do the
$PATH lookup in pure Perl to avoid doing the uncacheable lookup
in a vfork-ed child.