Perforce branch might be sync'ed multiple times with different

[trojanwarhorse1]

Perforce branch might be sync'ed multiple times with different
revision numbers (by the command "git p4 sync --branch=NAME //perforce/path...")

In such situation, git p4 will see multiple perforce branches with
same name and different revision numbers. The problem is that to make
a shelve, git-p4 script will try to find "origin" branch, if not
specified in command line explicitly. And previously script selected
any branch with same name and don't mention particular revision number.
Later this may cause failure of the command "git diff-tree -r $rev^ $rev",
so shelve can't be created (due to wrong origin branch/commit).

This commit fixes the heuristic by which git p4 selects origin branch:
first it tries to select branch with same perforce path and perforce
revision, and if it fails, then selects branch with only same perforce
path (ignoring perforce revision number).

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Originally posted by @kfrolov in git/git#1229