[PATCH] persistent-https: add go.mod to fix compile

From: Dominyk Tiller <dominyktiller@gmail.com>

GOPATH-based builds and non module-aware builds are being deprecated by golang upstream, which currently causes this to fail. This adds a barebones mod file to fix the build.

The `persistent-https` code hasn't been touched for a long time but I assume this is preferable to simply removing it from the codebase.

Before this change:
```
case $(go version) in \
	"go version go"1.[0-5].*) EQ=" " ;; *) EQ="=" ;; esac && \
	go build -o git-remote-persistent-https \
		-ldflags "-X main._BUILD_EMBED_LABEL${EQ}"
go: cannot find main module, but found .git/config in ../git
	to create a module there, run:
	cd ../.. && go mod init
make: *** [git-remote-persistent-https] Error 1
```

Ref: https://blog.golang.org/go116-module-changes.

Signed-off-by: Dominyk Tiller <dominyktiller@gmail.com>
---
    persistent-https: add go.mod to fix compile
    
    GOPATH-based builds and non module-aware builds are being deprecated by
    golang upstream, which currently causes this to fail. This adds a
    barebones mod file to fix the build.
    
    The persistent-https code hasn't been touched for a long time but I
    assume this is preferable to simply removing it from the codebase.
    Before this change:
    
    case $(go version) in \
        "go version go"1.[0-5].*) EQ=" " ;; *) EQ="=" ;; esac && \
        go build -o git-remote-persistent-https \
            -ldflags "-X main._BUILD_EMBED_LABEL${EQ}"
    go: cannot find main module, but found .git/config in ../git
        to create a module there, run:
        cd ../.. && go mod init
    make: *** [git-remote-persistent-https] Error 1
    
    
    Ref: https://blog.golang.org/go116-module-changes

Published-As: https://github.com/gitgitgadget/git/releases/tag/pr-git-989%2FDomT4%2Fgo_modules-v1
Fetch-It-Via: git fetch https://github.com/gitgitgadget/git pr-git-989/DomT4/go_modules-v1
Pull-Request: https://github.com/git/git/pull/989

 contrib/persistent-https/go.mod | 3 +++
 1 file changed, 3 insertions(+)
 create mode 100644 contrib/persistent-https/go.mod