libfetchers/git-utils: Be more correct about validating refnames

Turns out there's a much better API for this that doesn't have the
footguns of the previous method.

isLegalRefName is somewhat of a misnomer, since it's mainly used to
validate user inputs that can be either references, branch names,
psedorefs or tags.

(cherry picked from commit 5d1178b817)

tests/functional/fetchGitRefs.sh

@@ -59,6 +59,9 @@ invalid_ref() {
 }
 
 
+valid_ref 'A/b'
+valid_ref 'AaA/b'
+valid_ref 'FOO/BAR/BAZ'
 valid_ref 'foox'
 valid_ref '1337'
 valid_ref 'foo.baz'