fix(git): match git config values exactly when syncing (#39728) (#39729)

Backport #39728 by @silverwind

Since https://github.com/go-gitea/gitea/pull/39703, Gitea fails to start
when its gitconfig holds a `gc.reflogExpire` other than `90` (e.g.
`30.days`), because `git config --unset-all` exits 5 when no value
matches. Value patterns were also unanchored regexps, so `90` also
removed `90.days`.

- Match values exactly in `configUnsetAll` and `configAddNonExist`
- Treat "nothing to unset" as success
- Use `--replace-all` in `configSet` so a key with multiple values no
longer fails startup

Co-authored-by: silverwind <me@silverwind.io>

modules/git/config_test.go

@@ -32,7 +32,9 @@ func TestGitConfig(t *testing.T) {
 	assert.NoError(t, configSetNonExist(ctx, "test.key-a", "val-a-changed"))
 	assert.False(t, gitConfigContains("key-a = val-a-changed"))
 
+	assert.NoError(t, configAddNonExist(ctx, "test.key-a", "val-a2"))
 	assert.NoError(t, configSet(ctx, "test.key-a", "val-a-changed"))
+	assert.NoError(t, configUnsetAll(ctx, "test.key-a", "val-a"))
 	assert.True(t, gitConfigContains("key-a = val-a-changed"))
 
 	assert.NoError(t, configAddNonExist(ctx, "test.key-b", "val-b"))
@@ -41,6 +43,8 @@ func TestGitConfig(t *testing.T) {
 	assert.NoError(t, configAddNonExist(ctx, "test.key-b", "val-2b"))
 	assert.True(t, gitConfigContains("key-b = val-b"))
 	assert.True(t, gitConfigContains("key-b = val-2b"))
+	assert.NoError(t, configAddNonExist(ctx, "test.key-b", "val"))
+	assert.True(t, gitConfigContains("key-b = val\n"))
 
 	assert.NoError(t, configUnsetAll(ctx, "test.key-b", "val-b"))
 	assert.False(t, gitConfigContains("key-b = val-b"))