I recently tried installing the program via Homebrew on a machine that
didn't have 'libgit2' installed. The Homebrew formula installs pre-built
binaries. Running the binaries gave me an error saying that the
'libgit2' dynamic library couldn't be found.

Change the build configuration to build a "static" binary. It still
links to system dylibs, but I've removed all links to Homebrew dylibs.
These were:

* libgit2
* openssl

Removed the 'openssl' link by adding the 'vendored-openssl' feature to
the 'git2' dependency.

In order to remove the 'libgit2' link, I had to do some more finagling.
Accomplished this by setting an empty `PKG_CONFIG_LIBDIR` when building.
While 'git2' doesn't have a Cargo feature to disable dynamic linking, we
can force static linking by preventing `pkg-config` from finding the
Homebrew-installed 'libgit2'. The 'libgit2-sys' crate's build script
first checks for the library using `pkg-config`, and builds it
statically if it doesn't exist:

https://github.com/rust-lang/git2-rs/blob/43d583fd8b50fad0cfe0bbc334059ce965b6f3fc/libgit2-sys/build.rs#L15-L17

With these changes, the above packages will now be statically linked
with the program's binaries.

Upgrade 'git2' to the latest version (0.13.11) since we're changing its
build configuration.

Before:

    $ otool -L target/release/git-sug*
    target/release/git-sugapply:
      /System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 57740.60.18)
      /usr/local/opt/libgit2/lib/libgit2.1.0.dylib (compatibility version 1.0.0, current version 1.0.1)
      /usr/local/opt/openssl@1.1/lib/libssl.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
      /usr/local/opt/openssl@1.1/lib/libcrypto.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
      /usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.8)
      /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)
      /usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 1.0.0)
    target/release/git-sugapply.d: is not an object file
    target/release/git-sugdiff:
      /System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 57740.60.18)
      /usr/local/opt/libgit2/lib/libgit2.1.0.dylib (compatibility version 1.0.0, current version 1.0.1)
      /usr/local/opt/openssl@1.1/lib/libssl.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
      /usr/local/opt/openssl@1.1/lib/libcrypto.1.1.dylib (compatibility version 1.1.0, current version 1.1.0)
      /usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.8)
      /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)
      /usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 1.0.0)
    target/release/git-sugdiff.d: is not an object file

After:

    $ otool -L target/release/git-sug*
    target/release/git-sugapply:
      /System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 57740.60.18)
      /usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
      /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1349.8.0)
      /usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.8)
      /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)
      /usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 1.0.0)
    target/release/git-sugapply.d: is not an object file
    target/release/git-sugdiff:
      /System/Library/Frameworks/Security.framework/Versions/A/Security (compatibility version 1.0.0, current version 57740.60.18)
      /usr/lib/libiconv.2.dylib (compatibility version 7.0.0, current version 7.0.0)
      /System/Library/Frameworks/CoreFoundation.framework/Versions/A/CoreFoundation (compatibility version 150.0.0, current version 1349.8.0)
      /usr/lib/libz.1.dylib (compatibility version 1.0.0, current version 1.2.8)
      /usr/lib/libSystem.B.dylib (compatibility version 1.0.0, current version 1238.60.2)
      /usr/lib/libresolv.9.dylib (compatibility version 1.0.0, current version 1.0.0)
    target/release/git-sugdiff.d: is not an object file

This was replaced by `RELEASE_PRODUCTS` so that I could make
`DIST_PRODUCTS` based on `PRODUCTS`.

Add recipes to build release targets and copy the binaries and man pages
into the `dist/` folder. We'll use this to package a release archive.