Shallow clone by default (#122 #217)

[junegunn]

No description provided.

[junegunn]

--unshallow should be used only once. 😲

[junegunn]

And fetch --unshallow requires git 1.8.2 or above.

[junegunn]

And git fetch --depth 99999999 is slower than git fetch.

[junegunn]

So here's the summary of this PR:

1. Shallow clone (--depth 1)
   1. if g:plug_shallow is not set to 0 and tag is not specified.
   2. --no-single-branch option is also used for branch option
2. PlugUpdate will git fetch --depth 99999999 when tag option is given to "unshallow" the repository
   1. Not --unshallow because it requires git 1.8.2 or above
   2. --depth 99999999 is attached only when the repository is indeed shallow (.git/shallow exists) because depth makes fetch noticeably slower.